Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022	Dec. 31, 2021
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net income (loss) from discontinued operations, net of tax	$ 0.0	$ 6.4	$ 0.0	$ 10.1
Net (income) loss from discontinued operations attributable to noncontrolling interests	0.0	(0.5)	0.0	(0.6)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.0	(4.0)	0.0	(5.9)
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations			0.0	36.7
Net cash provided from (used for) investing activities			0.0	7.1
Net cash used from (used for) financing activities			0.0	(16.9)
Net change in cash during the period			(100.5)	235.7
Cash balances at beginning of period (includes restricted cash balances of $12.2 and $4.5 and excludes discontinued operations cash balances of $0.0 and $111.6)			255.0	147.7
Cash balances at end of period (includes restricted cash balances of $13.4 and $7.3 and excludes discontinued operations cash balances of $0.0 and $142.7)	154.5	383.4	154.5	383.4
Restricted cash balance	13.4	7.3	13.4	7.3	$ 12.2	$ 4.5
Interest paid			11.6	6.3
Net income tax payments			27.8	3.8
Discontinued Operations, Held-for-sale | NSM Insurance Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Commission revenues		80.2		150.3
Other revenues		22.7		41.1
Total revenues		102.9		191.4
General and administrative expenses		57.1		111.1
Broker commission expenses		24.1		44.7
Change in fair value of contingent consideration		0.0		0.1
Amortization of other intangible assets		0.0		9.1
Interest expense		8.4		10.5
Total expenses		89.6		175.5
Pre-tax income (loss) from discontinued operations		13.3		15.9
Income tax (expense) benefit		(6.9)		(5.8)
Net income (loss) from discontinued operations, net of tax		6.4		10.1
Net (income) loss from discontinued operations attributable to noncontrolling interests		(0.5)		(0.6)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		5.9		9.5
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		(4.0)		(5.9)
Comprehensive income (loss) from discontinued operations		1.9		3.6
Other comprehensive (income) loss from discontinued operations attributable to noncontrolling interests		0.2		0.3
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders		2.1		3.9
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations				36.7
Net cash provided from (used for) investing activities				7.1
Net cash used from (used for) financing activities				(16.9)
Effect of exchange rate changes on cash				4.2
Net change in cash during the period				31.1
Cash balances at beginning of period (includes restricted cash balances of $12.2 and $4.5 and excludes discontinued operations cash balances of $0.0 and $111.6)			0.0	111.6
Cash balances at end of period (includes restricted cash balances of $13.4 and $7.3 and excludes discontinued operations cash balances of $0.0 and $142.7)	$ 0.0	142.7	$ 0.0	142.7
Restricted cash balance		$ 108.9		108.9		$ 89.2
Interest paid				10.4
Net income tax payments				$ 0.0