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Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2022 | Sep. 30, 2021
Earnings Per Share [Abstract] |  |  |  |
Net income (losses) attributable to White Mountains’s common shareholders, net of restricted common share amounts | $ (4.7) | $ (372.1) | $ (148.3) | $ (300.7)
Dividends declared net of restricted common share amounts | 0.0 | 0.0 | (3.0) | (3.1)
Total undistributed net earnings (losses), net of restricted common share amounts | $ (4.7) | $ (372.1) | $ (151.3) | $ (303.8)

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- Definition Amount of paid and unpaid cash, stock, and paid-in-kind (PIK) dividends declared, for example, but not limited to, common and preferred stock. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI https://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 405 -Section 45 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=6957935&loc=d3e64057-112817 + Details
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- Definition The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 66 -URI https://asc.fasb.org/extlink&oid=126958026&loc=d3e2814-109256 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60B -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126958026&loc=SL5780133-109256 + Details
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- Definition Net Income (Loss) Attributable to Parent, Net of Restricted Common Share Amounts + References No definition available. + Details
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