UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q

WHITE MOUNTAINS INSURANCE GROUP, LTD.

(Exact name of registrant as specified in its charter)

80 SOUTH MAIN STREET, HANOVER, NEW HAMPSHIRE 03755-2053

(Address of principal executive offices including zip code)

(603) 640-2200

(Registrant’s telephone number, including area code)

Indicate by check mark whether the registrant: (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days

Yes  ý      No  o

Indicate by check mark whether the registrant is an accelerated filer (as defined in Rule 12b-2 of the Exchange Act)

Yes  ý      No  o

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act)

Yes  o      No  ý

As of October 28, 2005, 10,775,223 common shares with a par value of $1.00 per share (“Common Shares”) were outstanding (which includes 13,000 restricted Common Shares which were not vested at such date).

WHITE MOUNTAINS INSURANCE GROUP, LTD.

Table of Contents

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PART I. FINANCIAL INFORMATION.

Item 1. Financial Statements

WHITE MOUNTAINS INSURANCE GROUP, LTD.

CONSOLIDATED BALANCE SHEETS

Unaudited

See Notes to Consolidated Financial Statements

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WHITE MOUNTAINS INSURANCE GROUP, LTD.

CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME

Unaudited

See Notes to Consolidated Financial Statements

4

WHITE MOUNTAINS INSURANCE GROUP, LTD.

CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS’ EQUITY

Unaudited

See Notes to Consolidated Financial Statements

5

WHITE MOUNTAINS INSURANCE GROUP, LTD.

CONSOLIDATED STATEMENTS OF CASH FLOWS

Unaudited

See Notes to Consolidated Financial Statements

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NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (Unaudited)

Note 1. Basis of Presentation

These interim consolidated financial statements include the accounts of White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”) and its subsidiaries (collectively, “White Mountains”) and have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). The Company is a Bermuda exempted limited company with its headquarters located at the Bank of Butterfield Building, 42 Reid Street, Hamilton HM 12, Bermuda. The Company’s principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton HM 11, Bermuda. White Mountains’ reportable segments are OneBeacon, White Mountains Re, Esurance and Other Operations, as defined below.

The OneBeacon segment consists of the OneBeacon Insurance Group LLC family of companies (collectively “OneBeacon”), which are U.S.-based property and casualty insurance writers, substantially all of which operate in a multi-company pool. OneBeacon offers a wide range of specialty, personal and commercial products and services sold primarily through select independent agents. OneBeacon was acquired by White Mountains from Aviva plc (“Aviva”, formerly CGU) in 2001 (the “OneBeacon Acquisition”).

The White Mountains Re segment consists of White Mountains Re Group, Ltd. and its subsidiaries (collectively “White Mountains Re”). White Mountains Re offers lead reinsurance capacity for property, casualty, accident & health and marine exposures on a worldwide basis through its reinsurance subsidiaries, Folksamerica Reinsurance Company (“Folksamerica Re”, together with its parent, Folksamerica Holding Company, “Folksamerica”), which has been a wholly-owned subsidiary of White Mountains since 1998, and Sirius International Insurance Corporation (“Sirius International”), which has been a wholly-owned subsidiary of White Mountains since 2004. White Mountains’ reinsurance operations also include its wholly-owned subsidiaries, White Mountains Underwriting Limited (domiciled in Ireland) and White Mountains Underwriting (Bermuda) Limited (collectively, “WMU”). WMU is an underwriting advisory company specializing in international property and marine excess reinsurance.

The Esurance segment consists of Esurance Holdings, Inc. and its subsidiaries (collectively, “Esurance”). Esurance, which has been a unit of White Mountains since 2000, markets personal auto insurance directly to consumers online and through select online agents.

White Mountains’ Other Operations segment consists of the Company and its intermediate holding companies, as well as the International American Group, Inc. (the “International American Group”). The International American Group, which was acquired by White Mountains in 1999, consists of American Centennial Insurance Company (“American Centennial”) and British Insurance Company of Cayman (“British Insurance Company”), both of which are in run-off. The Other Operations segment also includes White Mountains’ investments in warrants to purchase common shares of both Montpelier Re Holdings, Ltd. (“Montpelier”) and Symetra Financial Corporation (“Symetra”).

White Mountains completed numerous significant transactions during 2004 that affect the comparability of the financial statement information presented herein. White Mountains’ consolidated statements of income and comprehensive income include the results of acquired businesses beginning as of the date each respective acquisition was completed.

All significant intercompany transactions have been eliminated in consolidation. The financial statements include all adjustments considered necessary by management to fairly present the financial position, results of operations and cash flows of White Mountains. All adjustments made are of a normal and recurring nature except as otherwise noted herein. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2004 Annual Report on Form 10-K. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Certain amounts in the prior period financial statements have been reclassified to conform with the current presentation. Refer to the Company’s 2004 Annual Report on Form 10-K for a complete discussion regarding White Mountains’ significant accounting policies.

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Recent Accounting Pronouncements

In December 2004, the Financial Accounting Standards Board (“FASB”) issued Statement of Financial Accounting Standards (“SFAS”) No. 123 (Revised 2004), “Share-Based Payment” (“SFAS 123(R)”). SFAS 123(R) is a revision of FASB Statement 123, “Accounting for Stock-Based Compensation” (“SFAS 123”) and supersedes APB Opinion No. 25, “Accounting for Stock Issued to Employees” (“APB 25”) and its related implementation guidance. The Statement focuses primarily on accounting for transactions in which an entity obtains employee services in share-based payment transactions. SFAS No. 123(R) requires a public entity to measure the cost of employee services received in exchange for an award of equity instruments based on the grant-date fair value of the award. That cost is to be recognized over the period during which an employee is required to provide service in exchange for the award.

On April 14, 2005, the Securities and Exchange Commission (the “SEC”) announced that the effective date of SFAS 123(R) will be suspended from June 15, 2005 until the first interim or annual reporting period that begins on or after December 15, 2005. White Mountains currently expects to adopt SFAS 123(R) effective January 1, 2006 and does not expect the adoption to have a material effect on its financial condition, results of operations or cash flows. White Mountains already expenses the full cost of all its share-based compensation, including its outstanding options to acquire Common Shares.

In June 2005, the FASB issued Statement No. 154, “Accounting Changes and Error Corrections” (“SFAS 154”), which replaces APB 20 and SFAS 3. SFAS 154 applies to all voluntary changes in accounting principles. The Statement requires that voluntary changes in accounting principles be applied retrospectively to prior periods unless doing so is impracticable. The statement does not change the transition method for new accounting standards where the new pronouncements address transition provisions, and accordingly, White Mountains does not expect that SFAS 154 will affect White Mountains’ adoption of SFAS 123(R).

Note 2.  Acquisitions and Dispositions

Sirius

On April 16, 2004, White Mountains acquired Sirius Insurance Holding Sweden AB and its subsidiaries (“Sirius”) from ABB Ltd. (the “Sirius Acquisition”) for SEK 3.27 billion (approximately $427.5 million based upon the foreign exchange spot rate at the date of acquisition), which included $10.5 million of expenses incurred in connection with the acquisition. The principal companies acquired were Sirius International, Sirius America Insurance Company (“Sirius America”) and Scandinavian Reinsurance Company Ltd. (“Scandinavian Re”). Sirius International is domiciled in Sweden and has offices in Belgium, Hamburg, London, Singapore, Stockholm and Zurich. Sirius America is a U.S.-based insurer focused on primary insurance programs that was acquired by Folksamerica as part of the transaction. Scandinavian Re is a reinsurance company that has been in run-off since 2002.

Supplemental unaudited pro forma condensed combined income statement information for the nine months ended September 30, 2004, which assumes that the Sirius Acquisition had occurred as of January 1, 2004, follows:

The unaudited pro forma information presented above for the nine months ended September 30, 2004 has been supplied for comparative purposes only and does not purport to reflect the actual results that would have been reported had the Sirius Acquisition been consummated at January 1, 2004. Additionally, such pro forma financial information does not purport to represent results that may occur in the future.

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Other

On September 30, 2005, OneBeacon sold National Farmers Union Property and Casualty Company (“NFU”) to QBE Insurance Group for $138.3 million in cash and recognized a gain of approximately $28 million ($23 million after-tax) on the sale through other revenues.

On September 30, 2005, Folksamerica sold one of its subsidiaries, California Indemnity Insurance Company (“CIIC”), for total proceeds of $19.8 million, $19.3 million of which was paid in cash, and recognized a gain of approximately $5.0 million ($3.3 million after-tax) on the sale through other revenues.

On August 2, 2005, OneBeacon sold one of its subsidiaries, Traders and Pacific Insurance Company (“TPIC”), to Endurance Reinsurance for $23.4 million in cash and recognized a gain of approximately $8 million ($5 million after-tax) on the sale through other revenues.

Note 3.  Loss and Loss Adjustment Expense Reserves

The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activities of White Mountains’ insurance and reinsurance subsidiaries for the three and nine months ended September 30, 2005 and 2004:

(1)   Reinsurance recoverables on unpaid losses acquired in the Sirius and Sierra Insurance Group acquisitions totalled $283.8 million and $162.5 million, respectively.

(2)   During the three and nine months ended September 30, 2005, White Mountains Re recorded $2 million and $8 million of unfavorable development on its workers compensation reserves, respectively, relating to its Sierra Insurance Group acquisition, which was offset dollar-for-dollar by a reduction in the principal amount of the adjustable note that White Mountains Re issued as part of the financing of that acquisition (See Note 6).

During the three and nine months ended September 30, 2005, White Mountains experienced $45.2 million and $43.9 million, respectively, of unfavorable development on prior accident year loss reserves. The majority of the adverse development resulted from a ground-up study of Folksamerica’s exposure to asbestos claims that was

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completed in the 2005 third quarter. The study examined losses incurred by all insureds that had reported over $250,000 of asbestos claims to Folksamerica and a significant sample of all other insureds with reported asbestos claims of less than $250,000. Comparing estimates generated by the study to Folksamerica’s exposed limits by underwriting year led to an increase of approximately $50 million in IBNR during the third quarter of 2005. Substantially all of this increase was offset by favorable activity recorded in the third quarter of 2005 related to the 2005 and 2004 underwriting years.

During the three and nine months ended September 30, 2004, White Mountains experienced $41.1 million and $72.2 million, respectively, of net unfavorable loss reserve development on prior accident year loss reserves, principally due to emerging claims experienced in OneBeacon’s run-off operations and increases in reserves at OneBeacon as a result of audits of national account and program claims administered by third parties.

In connection with purchase accounting for the acquisitions of OneBeacon and Sirius, White Mountains was required to adjust loss and LAE reserves and the related reinsurance recoverables to fair value on OneBeacon’s and Sirius’ acquired balance sheets. The net reduction to loss and LAE reserves is being recognized through an income statement charge ratably with and over the period the claims are settled. Accordingly, White Mountains recognized $9.1 million and $28.1 million of such charges, recorded as loss and LAE, for the three and nine months ended September 30, 2005, respectively, and $10.2 million and $33.1 million of such charges for the three and nine months ended September 30, 2004, respectively.

Note 4. Third Party Reinsurance

In the normal course of business, White Mountains’ insurance and reinsurance subsidiaries seek to limit losses that may arise from catastrophes or other events by reinsuring with third party reinsurers. White Mountains remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts.

OneBeacon

At September 30, 2005, OneBeacon had $29.0 million of reinsurance recoverables on paid losses and $3,380.5 million (gross of $265.6 million in purchase accounting adjustments, as described in Note 3) that will become recoverable if claims are paid in accordance with current reserve estimates. The collectibility of balances due from OneBeacon’s reinsurers is critical to OneBeacon’s financial strength because reinsurance contracts do not relieve OneBeacon of its primary obligation to its policyholders. OneBeacon is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. OneBeacon monitors the financial strength of its reinsurers on an ongoing basis. As a result, uncollectible amounts have historically not been significant. The following table provides a listing of OneBeacon’s top reinsurers based upon recoverable amounts, the percentage of total reinsurance recoverables and the reinsurers’ A.M. Best ratings.

(1)   OneBeacon also has an additional $449 million of Third Party Recoverables (as defined below) from various reinsurers, the majority of which are rated “A” or better by A.M. Best.

(2)   A.M. Best ratings as detailed above are: “A++” (Superior, which is the highest of fifteen ratings), “A+” ( Superior, which is the second highest of fifteen ratings) and “A” (Excellent, which is the third highest of fifteen ratings).

(3)   Includes $404.0 million of Third Party Recoverables, which NICO would pay under the terms of the NICO Cover (as defined below) if they are unable to collect from third party reinsurers.

(4)   At September 30, 2005, OneBeacon had assumed balances payable and expenses payable of approximately $15.1 million under its renewal rights agreement with Liberty Mutual Insurance Group (“Liberty Mutual”), which expired on October 31, 2003. In the event of a Liberty Mutual insolvency, OneBeacon has the right to offset these balances against its reinsurance recoverable due from Liberty Mutual.

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In June 2005, OneBeacon completed an internal study of its asbestos and environmental (“A&E”) exposures. Based on the results of the study, OneBeacon increased its best estimate of its incurred losses ceded to National Indemnity Company (“NICO”) by $353 million, net of underlying reinsurance ($841 million gross) to $2.1 billion. Due to the NICO Cover (as defined below), there was no impact to income or equity from the change in the estimate. See Item 2.  Management’s Discussion and Analysis of Financial Condition and Results of Operations - CRITICAL ACCOUNTING ESTIMATES for a detailed discussion of the results of this study.

In connection with the OneBeacon Acquisition, Aviva caused OneBeacon to purchase two reinsurance contracts: a full risk-transfer cover from NICO for up to $2.5 billion in old A&E claims and certain other exposures (the “NICO Cover”) and an adverse development cover from General Reinsurance Corporation (“GRC”) for up to $400.0 million of adverse development on losses occurring in years 2000 and prior (the “GRC Cover”) in addition to $170.0 million of reserves ceded as of the date of the OneBeacon Acquisition. The NICO Cover and GRC Cover, which were contingent on and occurred contemporaneously with the OneBeacon Acquisition, were put in place in lieu of a seller guarantee of loss and LAE reserves and are therefore accounted for as prospective reinsurance under GAAP in accordance with Emerging Issues Task Force Technical Matter Document No. D-54 (“EITF Topic
D-54”). NICO and GRC are wholly-owned subsidiaries of Berkshire Hathaway Inc. (“Berkshire”).

Under the terms of the NICO Cover, NICO receives reinsurance recoverables from certain of OneBeacon’s third party reinsurers (“Third Party Reinsurers”) in existence at the time the NICO Cover was executed (“Third Party Recoverables”). As a result, the Third Party Recoverables serve to protect the $2.5 billion limit of NICO coverage for the benefit of OneBeacon. White Mountains estimates that on an incurred basis, net of Third Party Recoverables, as of September 30, 2005 it has used approximately $2.1 billion of the coverage provided by NICO. Approximately $759 million of these incurred losses have been paid by NICO through September 30, 2005. At September 30, 2005, $31.7 million of the $2.1 billion of utilized coverage from NICO related to uncollectible Third Party Recoverables. To the extent that actual experience differs from White Mountains’ estimate of ultimate A&E losses and Third Party Recoverables, future losses could utilize some or all of the protection remaining under the NICO Cover.

Pursuant to the GRC Cover, OneBeacon is not entitled to recover losses to the full contract limit if such losses are reimbursed by GRC more quickly than anticipated at the time the contract was signed. OneBeacon will only seek reimbursement from GRC for claims which result in payment patterns similar to those supporting its recoverables recorded pursuant to the GRC Cover. The economic cost of not submitting certain other eligible claims to GRC is primarily the investment spread between the rate credited by GRC and the rate achieved by OneBeacon on its own investments. This cost, if any, is expected to be small.

White Mountains Re

At September 30, 2005, White Mountains Re had $74.0 million of reinsurance recoverables on paid losses and $1,693.2 million that will become recoverable if claims are paid in accordance with current reserve estimates. Because reinsurance contracts do not relieve White Mountains Re of its obligation to its ceding companies, the collectibility of balances due from its reinsurers is critical to White Mountains Re’s financial strength. White Mountains Re is selective with its reinsurers, placing reinsurance with only those reinsurers having strong financial condition. White Mountains Re monitors the financial strength of its reinsurers on an ongoing basis. Amounts due from certain of its reinsurers, including Olympus Reinsurance Company (“Olympus”), London Life & General Reinsurance Company Ltd. and London Life & Casualty Reinsurance Corp. (“London Life”) and Imagine Insurance Company, Limited (“Imagine Re”), are fully collateralized through funds held, letters of credit or trust agreements. The following table provides a listing of White Mountains Re’s top reinsurers based upon recoverable amounts, the percentage of total recoverables and the reinsurers’ A.M. Best ratings.

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(1)   A.M. Best ratings as detailed above are: “A++” (Superior, which is the highest of fifteen ratings), “A” (Excellent, which is the third highest of fifteen ratings), “A-” (Excellent, which is the fourth highest of fifteen ratings) and “B+” (Very Good, which is the sixth highest of fifteen ratings).

At September 30, 2005, White Mountains Re had $705.6 million of reinsurance recoverables from Olympus. Included in this amount is $457.2 million recorded in the third quarter of 2005 related to hurricanes Katrina and Rita. As a result of these property catastrophe events in the third quarter of 2005, Olympus’ rating was downgraded by A.M. Best from A- to B+. White Mountains Re’s reinsurance recoverables from Olympus are fully collateralized, in the form of assets in a trust, funds held and offsetting balances payable. During October 2005, Olympus provided additional collateral to White Mountains Re, which increased their collateralized position to 115% of the balance of reinsurance recoverable from Olympus.

Note 5.  Investment Securities

White Mountains’ net investment income is comprised primarily of interest income associated with White Mountains’ fixed maturity investments, dividend income from its equity investments and interest income from its short-term investments. Net investment income for the three and nine months ended September 30, 2005 and 2004 consisted of the following:

During the first quarter of 2005, Montpelier declared a special dividend of $5.50 per share, payable to holders of both its common shares and warrants to acquire its common shares. White Mountains recorded pre-tax investment income of $74.1 million in the first quarter for this special dividend, of which $34.7 million (relating to its common share investment) was included in net investment income from common equity securities and $39.4 million (relating to its warrant investment) was included in net investment income from other investments. For the three and nine months ended September 30, 2005, White Mountains also recorded an aggregate of $4.9 million and $14.7 million, respectively, in pre-tax investment income from Montpelier’s regular quarterly dividend.

The composition of realized investment gains (losses) consisted of the following: