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Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Aug. 01, 2022 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2018 | Dec. 31, 2020
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal, Statement of Income or Comprehensive Income [Extensible Enumeration] |  |  | General and administrative expenses | General and administrative expenses |  |
Net income (loss) from discontinued operations, net of tax - NSM Group |  | $ 0.0 | $ 16.4 | $ (22.6) |  |
Net (income) loss from discontinued operations attributable to noncontrolling interests |  | 0.0 | (0.7) | 1.0 |  |
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders |  | 0.0 | 903.2 | (3.9) |  |
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group |  | 0.0 | (5.2) | 0.2 |  |
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group |  | 0.0 | 2.9 | 0.0 |  |
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest |  | (0.5) | 0.9 | 0.2 |  |
Net Cash Provided by (Used in) Discontinued Operations [Abstract] |  |  |  |  |  |
Net cash provided from (used for) operations |  | 0.0 | 38.7 | 42.3 |  |
Net cash provided from (used for) investing activities |  | 0.0 | 7.1 | (56.5) |  |
Net cash used from (used for) financing activities |  | 0.0 | (17.5) | (1.0) |  |
Cash balances at beginning of period (includes restricted cash of $89.2 and $78.4 ) |  | 0.0 | 111.6 | 126.6 |  |
Cash balances at end of period (includes restricted cash of $0.0 and $89.2) |  | 0.0 | 0.0 | 111.6 |  |
NSM |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Net gain (loss) from sale of discontinued operations, net of tax |  | 0.0 | 886.8 | 0.0 |  |
Sirius Group |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Net gain (loss) from sale of discontinued operations, net of tax |  | 0.0 | 0.0 | 18.7 |  |
Discontinued Operations, Disposed of by Sale | NSM |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Total revenues - NSM Group |  |  | 225.0 | 330.4 |  |
General and administrative expenses | $ 14.0 |  | 126.8 | 190.4 |  |
Broker commission expenses |  |  | 52.9 | 80.2 |  |
Change in fair value of contingent consideration |  |  | 0.1 | 1.0 |  |
Amortization of other intangible assets |  |  | 9.1 | 35.2 |  |
Loss on assets held for sale |  |  | 0.0 | 28.7 |  |
Interest expense |  |  | 12.1 | 23.3 |  |
Total expenses - NSM Group |  |  | 201.0 | 358.8 |  |
Pre-tax income (loss) from discontinued operations - NSM Group |  |  | 24.0 | (28.4) |  |
Income tax (expense) benefit |  |  | (7.6) | 5.8 |  |
Net income (loss) from discontinued operations, net tax - NSM Group |  |  | 16.4 | (22.6) |  |
Net gain (loss) from sale of discontinued operations, net of tax | 886.8 |  | 886.8 | 0.0 |  |
Net income (loss) from discontinued operations, net of tax - NSM Group |  |  | 903.2 | (3.9) |  |
Net (income) loss from discontinued operations attributable to noncontrolling interests |  |  | (0.7) | 1.0 |  |
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders |  |  | 902.5 | (2.9) |  |
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group |  |  | (5.2) | 0.2 |  |
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group | $ 2.9 |  | 2.9 | 0.0 |  |
Comprehensive income (loss) attributable to White Mountains’s common shareholders |  |  | 900.2 | (2.7) |  |
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest |  |  | 0.2 | (0.1) |  |
Comprehensive income (loss) attributable to White Mountains’s common shareholders |  |  | 900.4 | (2.8) |  |
Net Cash Provided by (Used in) Discontinued Operations [Abstract] |  |  |  |  |  |
Net cash provided from (used for) operations |  |  | 38.7 | 42.3 |  |
Net cash provided from (used for) investing activities |  |  | 7.1 | (56.5) |  |
Net cash used from (used for) financing activities |  |  | (17.5) | (1.0) |  |
Effect of exchange rate changes on cash |  |  | 4.0 | 0.2 |  |
Net change in cash during the period |  |  | 32.3 | (15.0) |  |
Cash balances at beginning of period (includes restricted cash of $89.2 and $78.4 ) |  | $ 0.0 | 111.6 | 126.6 |  |
Restricted cash balance |  |  | 0.0 | 89.2 |  | $ 78.4
Cash sold as part of the sale of NSM Group (includes restricted cash of $105.1 and $0.0) |  |  | (143.9) | 0.0 |  |
Restricted cash |  |  | 105.1 | 0.0 |  |
Cash balances at end of period (includes restricted cash of $0.0 and $89.2) |  |  | 0.0 | 111.6 |  |
Interest paid |  |  | (12.0) | (16.6) |  |
Net income tax payments |  |  | 0.0 | 0.0 |  |
Discontinued Operations, Disposed of by Sale | NSM | Commission revenues |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Total revenues - NSM Group |  |  | 176.9 | 258.0 |  |
Discontinued Operations, Disposed of by Sale | NSM | Other revenues |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Total revenues - NSM Group |  |  | 48.1 | 72.4 |  |
Discontinued Operations, Disposed of by Sale | Sirius Group |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] |  |  |  |  |  |
Net gain (loss) from sale of discontinued operations, net of tax |  |  |  | 17.6 | $ (17.3) |
Net gain (loss) from sale of discontinued operations, net of tax |  |  | $ 0.0 | $ 18.7 |  |

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