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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, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Earnings Per Share [Abstract] |  |  |  |
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts | $ 230.4 | $ 47.1 | $ 217.4 | $ 347.6
Dividends declared net of restricted common share amounts | 0.0 | 0.0 | (3.1) | (3.2)
Total undistributed net earnings, net of restricted common share amounts | $ 230.4 | $ 47.1 | $ 214.3 | $ 344.4

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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 http://asc.fasb.org/extlink&oid=121326447&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 http://asc.fasb.org/extlink&oid=121326447&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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