Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 01, 2022	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2018	Dec. 31, 2021	Dec. 31, 2020
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net income (loss) from discontinued operations, net of tax - NSM Group		$ 6.3	$ 5.3	$ 16.4	$ (23.8)
Net gain (loss) from sale of discontinued operations, net of tax				886.8	18.7
Total income (loss) from discontinued operations, net of tax		(893.1)	(5.3)	(903.2)	5.1
Net (income) loss from discontinued operations attributable to non-controlling interests		(0.1)	0.0	(0.7)	1.1
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		0.7	(2.2)	(5.2)	0.9
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		2.9	0.0	2.9	0.0
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations				38.7	40.3
Net cash provided from (used for) investing activities				7.1	(54.6)
Net cash used from (used for) financing activities				(17.5)	13.8
Net change in cash during the period				98.0	99.4
Cash balances at beginning of period (includes restricted cash balances of $4.5 and $0.0 and excludes discontinued operations cash balances of $111.6 and $126.6)				147.7	84.6
Cash balances at end of period (includes restricted cash balances of $8.9 and $4.0 and excludes discontinued operations cash balances of $0.0 and 126.7)		245.7	184.0	245.7	184.0
Restricted cash balance		8.9	4.0	8.9	4.0		$ 4.5	$ 0.0
Interest paid				(10.9)	(5.3)
Net income tax payments				3.2	2.1
NSM Insurance Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax		886.8	0.0	886.8	0.0
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash sold as part of the sale of NSM Group (includes restricted cash of $105.1 and $0.0)				143.9	0.0
Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax		0.0	0.0	0.0	18.7	$ 17.3
Discontinued Operations, Held-for-sale | NSM Insurance Group
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Short-term investments, at fair value							7.8
Cash (restricted $89.2)							111.6
Restricted cash							89.2
Premiums and commissions receivable							85.0
Goodwill and other intangible assets							725.4
Other assets							59.2
Total assets held for sale							989.0
Debt							272.1
Premiums payable							135.9
Contingent consideration							6.8
Other liabilities							80.5
Total liabilities held for sale							495.3
Net assets held for sale							493.7
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Commission revenues		26.6	67.0	176.9	194.6
Other revenues		7.0	15.3	48.1	46.8
Total revenues - NSM Group		33.6	82.3	225.0	241.4
General and administrative expenses		15.7	48.9	126.8	142.4
Broker commission expenses		8.2	20.4	52.9	60.9
Change in fair value of contingent consideration		0.0	0.6	0.1	0.8
Amortization of other intangible assets		0.0	8.2	9.1	25.0
Loss on assets held for sale		0.0	0.0	0.0	(28.7)
Interest expense		1.6	5.9	12.1	17.7
Total expenses - NSM Group		25.5	84.0	201.0	275.5
Pre-tax income (loss) from discontinued operations - NSM Group		8.1	(1.7)	24.0	(34.1)
Income tax (expense) benefit		(1.8)	7.0	(7.6)	10.3
Net income (loss) from discontinued operations, net of tax - NSM Group		6.3	5.3	16.4	(23.8)
Net gain (loss) from sale of discontinued operations, net of tax		886.8	0.0	886.8	0.0
Total income (loss) from discontinued operations, net of tax		893.1	5.3	903.2	(5.1)
Net (income) loss from discontinued operations attributable to non-controlling interests		(0.2)	0.0	(0.8)	1.1
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		892.9	5.3	902.4	(4.0)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		0.7	(1.7)	(5.2)	0.9
Comprehensive income (loss) from discontinued operations		896.5	3.6	900.1	(3.1)
Other comprehensive income (loss) from discontinued operations attributable to non-controlling interests		(0.1)	0.0	0.2	(0.1)
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders		896.4	3.6	900.3	(3.2)
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations				38.7	40.3
Net cash provided from (used for) investing activities				7.1	(54.6)
Net cash used from (used for) financing activities				(17.5)	13.8
Effect of exchange rate changes on cash				4.0	0.6
Net change in cash during the period				32.3	0.1
Cash balances at beginning of period (includes restricted cash balances of $4.5 and $0.0 and excludes discontinued operations cash balances of $111.6 and $126.6)				111.6	126.6
Cash sold as part of the sale of NSM Group (includes restricted cash of $105.1 and $0.0)				(143.9)	0.0
Cash balances at end of period (includes restricted cash balances of $8.9 and $4.0 and excludes discontinued operations cash balances of $0.0 and 126.7)		0.0	126.7	0.0	126.7
Restricted cash balance		0.0	92.2	0.0	92.2		$ 89.2	78.4
Interest paid				(12.0)	(16.6)
Net income tax payments				0.0	0.0
Discontinued Operations, Held-for-sale | Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax		0.0	0.0	0.0	18.7
Discontinued Operations, Disposed of by Sale | NSM Insurance Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Loss on assets held for sale	$ (875.7)
Net gain (loss) from sale of discontinued operations, net of tax	886.8
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	$ 2.9
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Restricted cash balance		$ 105.1	$ 0.0	$ 105.1	0.0
Discontinued Operations, Disposed of by Sale | Sirius Group
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Total liabilities held for sale								$ 18.7
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain (loss) from sale of discontinued operations, net of tax					$ 17.6