Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ (290.6)	$ 701.7	$ 408.4
Dividends declared, net of restricted common share amounts	(3.1)	(3.1)	(3.2)
Total undistributed net earnings (losses), net of restricted common share amounts	$ (293.7)	$ 698.6	$ 405.2

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.

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=125511455&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=125511455&loc=SL5780133-109256

Name:	us-gaap_UndistributedEarnings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration