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SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION | 12 Months Ended
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Dec. 31, 2023
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 Segment |  | 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, 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
| HG Global/BAM | $ | 40.1 |  |  | $ | — |  |  | $ | 325.8 |  |  | $ | — |  |  | $ | 31.2 |  |  | $ | 31.7 |  |  | $ | — |  |  | $ | 8.6 |  |  | $ | .4 |  |  | $ | 58.6 |
| Ark/WM Outrigger | 145.3 |  |  | 1,605.1 |  |  | 743.6 |  |  | — |  |  | 1,409.7 |  |  | 61.4 |  |  | 726.8 |  |  | 273.1 |  |  | 113.6 |  |  | 1,520.9 |
December 31, 2022 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
| HG Global/BAM | 36.0 |  |  | — |  |  | 298.3 |  |  | — |  |  | 33.3 |  |  | 21.5 |  |  | — |  |  | 11.2 |  |  | .4 |  |  | 65.1 |
| Ark/WM Outrigger |  | 127.2 |  |  | 1,296.5 |  |  | 623.2 |  |  | — |  |  | 1,043.4 |  |  | 16.3 |  |  | 536.4 |  |  | 238.3 |  |  | 78.7 |  |  | 1,195.2 |
December 31, 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
| HG Global/BAM | 33.1 |  |  | — |  |  | 266.3 |  |  | — |  |  | 26.9 |  |  | 17.5 |  |  | — |  |  | 8.3 |  |  | .4 |  |  | 55.8 |
| Ark/WM Outrigger |  | 100.8 |  |  | 894.7 |  |  | 495.9 |  |  | — |  |  | 637.3 |  |  | 2.9 |  |  | 314.8 |  |  | 111.3 |  |  | 64.6 |  |  | 859.1 |
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 -Publisher SEC Reference 2: 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//1943274/2147480149/944-235-S99-1 Reference 3: 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//1943274/2147480149/944-235-S99-1 + Details
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