0001628280-26-046118 | 11-K | XBRL Viewer | White Mountains Insurance Group LTD

Cover	12 Months Ended
Dec. 31, 2025
Document Information [Line Items]
Document Type	11-K
Entity Registrant Name	White Mountains Insurance Group, Ltd
Entity Central Index Key	0000776867
Amendment Flag	false
EBP 001
Document Information [Line Items]
Document Type	11-K
Entity Registrant Name	White Mountains Insurance Group, Ltd
Entity Central Index Key	0000776867
Amendment Flag	false

Basic Financial Information	12 Months Ended
Dec. 31, 2025
EBP 001
EBP, Description of Plan [Line Items]
Basic Financial Information	Basic Financial Statement Benefits under the plan are provided by a trust fund. Plan expenses were $763,201. These expenses included $1,324 in administrative expenses and $761,877 in benefits paid to participants and beneficiaries, and $0 in other expenses. A total of 95 persons were participants in or beneficiaries of the plan at the end of the plan year, although not all of these persons had yet earned the right to receive benefits. The value of plan assets, after subtracting liabilities of the plan, was $56,845,596 as of the end of the plan year, compared to $47,599,790 as of the beginning of the plan year. During the plan year the plan experienced a change in its net assets of $9,245,806. This change includes unrealized appreciation or depreciation in the value of plan assets; that is, the difference between the value of the plan's assets at the end of the year and the value of the assets at the beginning of the year or the cost of assets acquired during the year. The plan had total income of $10,009,007, including employer contributions of $1,169,874, employee contributions of $1,570,203 and earnings from investments of $7,254,335.

Schedule I (Form 5500) (Tables)	12 Months Ended
Dec. 31, 2025
EBP 001
EBP, Reconciliation of Financial Statement to Form 5500 [Line Items]
Schedule I (Form 5500)

v3.26.1

Schedule I (Form 5500) (Details) - EBP 001 - USD ($)	12 Months Ended
Schedule I (Form 5500) (Details) - EBP 001 - USD ($)	Dec. 31, 2025	Dec. 31, 2024
EBP, Reconciliation of Financial Statement to Form 5500 [Line Items]
Total plan assets	$ 56,845,596	$ 47,599,790
Total plan liabilities	0	0
Net plan assets	56,845,596	$ 47,599,790
Contributions receivable, employers	1,169,874
Contributions receivable, participants	1,570,203
Contributions receivable, others	14,595
Other income	7,254,335
Total income	10,009,007
Benefits paid (including direct rollovers)	761,877
Administrative service providers (salaries, fees, and commissions)	1,324
Total expenses	763,201
Net income (loss)	9,245,806
Employer securities	7,658,080
Participant loans	100,523
Fidelity bond	$ 30,000,000
Entity Tax Identification Number	57-1163892
Plan number	001

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v3.26.1

Basic Financial Information (Details) - EBP 001	12 Months Ended
Basic Financial Information (Details) - EBP 001	Dec. 31, 2025 USD ($) participant	Dec. 31, 2024 USD ($)
EBP, Description of Plan [Line Items]
Expenses	$ 763,201
Administrative expenses	1,324
Benefits paid to participants and beneficiaries	761,877
Other expenses	$ 0
Number of participants \| participant	95
Net plan assets	$ 56,845,596	$ 47,599,790
Change in net asset available for benefit	9,245,806
Income	10,009,007
Employer contributions	1,169,874
Employee contributions	1,570,203
Earnings from investments	$ 7,254,335

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-Paragraph 7
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477063/962-205-45-7

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Print Document
Reports Cover Basic Financial Information Schedule I (Form 5500) Tables Schedule I (Form 5500) (Tables) Details Basic Financial Information (Details) Schedule I (Form 5500) (Details) All Reports

Schedule I (Form 5500)	12 Months Ended
Schedule I (Form 5500)	Dec. 31, 2025
EBP 001
EBP, Reconciliation of Financial Statement to Form 5500 [Line Items]
Schedule I (Form 5500)