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SCHEDULE VI - SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS | 12 Months Ended
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Dec. 31, 2025
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract] |
SCHEDULE VI - SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS | WHITE MOUNTAINS INSURANCE GROUP, LTD. SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Millions)
Column A |  | Column B |  | Column C |  | Column D |  | Column E |  | Column F |  | Column G |  | Column H |  | Column I |  | Column J |  | Column K
|  | Deferred acquisition costs |  | Reserves for Unpaid Claims and Claims Adjustment Expenses |  | Discount, if any, deducted in Column C |  | Unearned Premiums |  | Earned Premiums |  | Net investment income |  | Claims and Claims Adjustment Expenses Incurred Related to |  | Amortization of deferred policy acquisition costs |  | Paid Claims and Claims Adjustment Expenses |  | Net Premiums written
Affiliation with registrant |  |  |  |  |  |  |  | Current Year |  | Prior Year |  |  |
Ark/WM Outrigger |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2025 |  | $ | 211.1 |  |  | $ | 2,481.0 |  |  | $ | — |  |  | $ | 1,026.1 |  |  | $ | 1,697.4 |  |  | $ | 106.5 |  |  | $ | 956.9 |  |  | $ | (112.7) |  |  | $ | 374.4 |  |  | $ | 585.7 |  |  | $ | 1,811.6 |
2024 |  | 165.2 |  |  | 2,127.5 |  |  | — |  |  | 853.3 |  |  | 1,587.8 |  |  | 90.7 |  |  | 914.4 |  |  | (58.6) |  |  | 294.1 |  |  | 416.7 |  |  | 1,679.1 |
2023 |  | 145.3 |  |  | 1,605.1 |  |  | — |  |  | 743.6 |  |  | 1,409.7 |  |  | 61.4 |  |  | 706.9 |  |  | 19.9 |  |  | 273.1 |  |  | 407.9 |  |  | 1,520.9 |
Bamboo Captive (1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2025 |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 26.7 |  |  | $ | 1.8 |  |  | $ | 16.0 |  |  | $ | 2.0 |  |  | $ | 9.7 |  |  | $ | 27.2 |  |  | $ | 16.1 |
2024 |  | 10.2 |  |  | 17.8 |  |  | — |  |  | 31.5 |  |  | 39.4 |  |  | 1.4 |  |  | 17.3 |  |  | 3.3 |  |  | 14.1 |  |  | 12.0 |  |  | 50.0 |
2023 |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Bamboo CRVs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2025 |  | $ | 2.5 |  |  | $ | 13.6 |  |  | $ | — |  |  | $ | 9.6 |  |  | $ | 22.2 |  |  | $ | 1.5 |  |  | $ | 16.2 |  |  | $ | 3.8 |  |  | $ | 8.3 |  |  | $ | 16.4 |  |  | $ | 1.4 |
2024 |  | 5.8 |  |  | 12.1 |  |  | — |  |  | 29.0 |  |  | 32.7 |  |  | .4 |  |  | 12.1 |  |  | — |  |  | 12.1 |  |  | — |  |  | 61.7 |
2023 |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
(1) The Bamboo Captive’s results are from January 1, 2025 through December 5, 2025, the date of sale.

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- Definition The entire disclosure of supplemental information for registrant, its subsidiary and 50-percent-or-less-owned equity basis investee, with liability for property-casualty insurance claim. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column A -Subparagraph (c) -Footnote 3 -Publisher SEC Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Publisher SEC Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column A)(c)(Footnote 3)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
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