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SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION | 12 Months Ended
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Dec. 31, 2025
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Abstract] |
SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION | WHITE MOUNTAINS INSURANCE GROUP, LTD. SUPPLEMENTARY INSURANCE INFORMATION
| Column A |  | Column B |  | Column C |  | Column D |  | Column E |  | Column F |  | Column G |  | Column H |  | Column I |  | Column J |  | Column K
| Millions |  | Deferred Acquisition Costs |  | Future Policy Benefits, Losses, Claims and Loss Expenses |  | Unearned Premiums |  | Other Policy Claims and Benefits Payable |  | Premiums Earned |  | Net Investment Income |  | Benefits, Claims, Losses and Settlement Expenses |  | Amortization of Deferred Acquisition Costs |  | Other Operating Expenses |  | Premiums Written
Years ended: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
December 31, 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
| Ark/WM Outrigger | $ | 211.1 |  |  | $ | 2,481.0 |  |  | $ | 1,026.1 |  |  | $ | — |  |  | $ | 1,697.4 |  |  | $ | 106.5 |  |  | $ | 844.2 |  |  | $ | 374.4 |  |  | $ | 143.9 |  |  | $ | 1,811.6 |
| HG Global | 96.9 |  |  | — |  |  | 327.9 |  |  | — |  |  | 30.8 |  |  | 27.1 |  |  | — |  |  | 8.0 |  |  | — |  |  | 61.4 |
| Bamboo Captive (1) |  | — |  |  | — |  |  | — |  |  | — |  |  | 26.7 |  |  | 1.8 |  |  | 18.0 |  |  | 9.7 |  |  | — |  |  | 16.1 |
| Bamboo CRV | 2.5 |  |  | 13.6 |  |  | 9.6 |  |  | — |  |  | 22.2 |  |  | 1.5 |  |  | 20.0 |  |  | 8.3 |  |  | — |  |  | 1.4 |
December 31, 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
| Ark/WM Outrigger |  | 165.2 |  |  | 2,127.5 |  |  | 853.3 |  |  | — |  |  | 1,587.8 |  |  | 90.7 |  |  | 855.8 |  |  | 294.1 |  |  | 136.1 |  |  | 1,679.1 |
| HG Global | 86.6 |  |  | — |  |  | 297.3 |  |  | — |  |  | 31.7 |  |  | 32.2 |  |  | — |  |  | 8.2 |  |  | .2 |  |  | 56.0 |
| Bamboo Captive |  | 10.2 |  |  | 17.8 |  |  | 31.5 |  |  | — |  |  | 39.4 |  |  | 1.4 |  |  | 20.6 |  |  | 14.1 |  |  | — |  |  | 50.0 |
| Bamboo CRV | 5.8 |  |  | 12.1 |  |  | 29.0 |  |  | — |  |  | 32.7 |  |  | .4 |  |  | 12.1 |  |  | 12.1 |  |  | — |  |  | 61.7 |
December 31, 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
| Ark/WM Outrigger |  | 145.3 |  |  | 1,605.1 |  |  | 743.6 |  |  | — |  |  | 1,409.7 |  |  | 61.4 |  |  | 726.8 |  |  | 273.1 |  |  | 113.6 |  |  | 1,520.9 |
| HG Global | 40.1 |  |  | — |  |  | 325.8 |  |  | — |  |  | 31.2 |  |  | 31.7 |  |  | — |  |  | 8.6 |  |  | .4 |  |  | 58.6 |
(1) The Bamboo Captive’s results are from January 1, 2025 through December 5, 2025, the date of sale. Schedules of the Registrant should be read in conjunction with the Consolidated Financial Statements and Notes.

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- Definition The entire disclosure for supplementary insurance information by segment. Includes, but is not limited to, deferred policy acquisition cost; future policy benefit, loss, claim and loss expenses; unearned premium; other policy claim and benefit payable; premium revenue; net investment income; benefit, claim, loss and settlement expenses; amortization of deferred policy acquisition cost; other operating expense; and premium written. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 16 -Paragraph Column J -Footnote 3 -Publisher SEC Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 16 -Paragraph Column G -Footnote 3 -Publisher SEC Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16(Column G)(Footnote 3)) -SubTopic 235 -Topic 944 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16(Column J)(Footnote 3)) -SubTopic 235 -Topic 944 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-1 + Details
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