Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions		12 Months Ended
	Aug. 01, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		$ 0.0	$ 0.0	$ 903.2
Net (income) loss from discontinued operations attributable to noncontrolling interests		0.0	0.0	(0.7)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders				902.5
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		0.0	0.0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		0.0	0.0	2.9
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest		0.0	(0.5)	0.9
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations		0.0	0.0	38.7
Net cash provided from (used for) investing activities		0.0	0.0	7.1
Net cash used from (used for) financing activities		0.0	0.0	(17.5)
Cash balances at beginning of period (includes restricted cash of $89200000)		0.0	0.0	116.6
Cash balances at end of period (includes restricted cash of $0.0)		0.0	0.0	0.0
NSM
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group		$ 0.0	0.0	886.8
Discontinued Operations, Disposed of by Sale | NSM
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues - NSM Group				225.0
General and administrative expenses	$ 14.0			126.8
Broker commission expenses				52.9
Change in fair value of contingent consideration				0.1
Amortization of other intangible assets				9.1
Interest expense				12.1
Total expenses - NSM Group				201.0
Pre-tax income (loss) from discontinued operations - NSM Group				24.0
Income tax (expense) benefit				(7.6)
Net income (loss) from discontinued operations, net tax - NSM Group				16.4
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	886.8			886.8
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders				903.2
Net (income) loss from discontinued operations attributable to noncontrolling interests				(0.7)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders				902.5
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group				(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	$ 2.9			2.9
Comprehensive income (loss) attributable to White Mountains’s common shareholders				900.2
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest				0.2
Comprehensive income (loss) attributable to White Mountains’s common shareholders				900.4
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				0.0	$ 89.2
Cash balances at beginning of period (includes restricted cash of $89200000)			$ 0.0	111.6
Cash sold as part of the sale of NSM Group (includes restricted cash of $105100000)				(143.9)
Cash balances at end of period (includes restricted cash of $0.0)				0.0
Restricted cash				105.1
Interest paid				(12.0)
Net income tax payments				0.0
Discontinued Operations, Disposed of by Sale | NSM | Commission revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues - NSM Group				176.9
Discontinued Operations, Disposed of by Sale | NSM | Other revenues
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total revenues - NSM Group				$ 48.1