Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Aug. 01, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group		$ 0.0	$ 886.8	$ 0.0	$ 886.8
Net income (loss) from discontinued operations, net of tax - NSM Group		0.0	6.3	0.0	16.4
Net (income) loss from discontinued operations attributable to noncontrolling interests		0.0	(0.1)	0.0	(0.7)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		0.0	893.1	0.0	903.2
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		0.0	0.7	0.0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		0.0	2.9	0.0	2.9
Total comprehensive income (loss)		21.9	890.4	223.6	747.1
Other comprehensive (income) loss from discontinued operations attributable to noncontrolling interests		0.7	(0.3)	(0.1)	0.4
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders		22.6	890.1	223.5	747.5
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations				0.0	38.7
Net cash provided from (used for) investing activities				0.0	7.1
Net cash used from (used for) financing activities				0.0	(17.5)
Restricted cash balance		0.0	8.9	0.0	8.9	$ 12.2	$ 4.5
Discontinued Operations, Disposed of by Sale | NSM Insurance Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues			33.6		225.0
General and administrative expenses	$ 14.0		15.7		126.8
Broker commission expenses			8.2		52.9
Change in fair value of contingent consideration			0.0		0.1
Amortization of other intangible assets			0.0		9.1
Interest expense			1.6		12.1
Total expenses			25.5		201.0
Pre-tax income (loss) from discontinued operations			8.1		24.0
Income tax (expense) benefit			(1.8)		(7.6)
Net income (loss) from discontinued operations, net of tax - NSM Group			6.3		16.4
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	886.8		886.8		886.8
Net income (loss) from discontinued operations, net of tax - NSM Group			893.1		903.2
Net (income) loss from discontinued operations attributable to noncontrolling interests			(0.2)		(0.8)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders			892.9		902.4
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group			0.7		(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	$ 2.9		2.9		2.9
Total comprehensive income (loss)			896.5		900.1
Other comprehensive (income) loss from discontinued operations attributable to noncontrolling interests			(0.1)		0.2
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders			896.4		900.3
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations					38.7
Net cash provided from (used for) investing activities					7.1
Net cash used from (used for) financing activities					(17.5)
Effect of exchange rate changes on cash					4.0
Net change in cash during the period					32.3
Restricted cash balance							$ 89.2
Cash balances at beginning of period (includes restricted cash of $89.2)				0.0	111.6
Cash sold as part of the sale of NSM Group			(143.9)		(143.9)
Cash balances at end of period		$ 0.0	0.0	$ 0.0	0.0
Interest paid					(12.0)
Net income tax payments					0.0
Discontinued Operations, Disposed of by Sale | NSM Insurance Group | Commission revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues			26.6		176.9
Discontinued Operations, Disposed of by Sale | NSM Insurance Group | Other revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues			$ 7.0		$ 48.1