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Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net loss attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ (93.1)	$ (128.4)
Dividends declared net of restricted common share amounts	(3.1)	(3.1)
Total undistributed net losses, net of restricted common share amounts	$ (96.2)	$ (131.5)

X

- 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)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 45
-Paragraph 2
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- References

No definition available.

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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

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 45
-Paragraph 60B
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- Definition

Net Income (Loss) Attributable to Parent, Net of Restricted Common Share Amounts

+ References

No definition available.

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