Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions		12 Months Ended
	Aug. 01, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2018	Dec. 31, 2019
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net income (loss) from discontinued operations, net of tax - NSM Group		$ 16.4	$ (22.6)	$ (9.5)
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		(903.2)	3.9	11.8
Net (income) loss from discontinued operations attributable to non-controlling interests		(0.7)	1.0	0.2
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		(5.2)	0.2	5.9
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group		2.9	0.0	0.0
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations		38.7	42.3	35.5
Net cash provided from (used for) investing activities		7.1	(56.5)	(124.9)
Net cash used from (used for) financing activities		(17.5)	(1.0)	128.8
Net decrease in cash during the year		107.3	63.1	13.6
Cash balance at beginning of year		147.7	84.6	71.0
Cash balance at end of year		255.0	147.7	84.6
Net income tax payments to US		10.3	(0.1)	15.9
NSM
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net income (loss) from discontinued operations, net of tax - NSM Group		16.4	(22.6)	(9.5)
Gain (loss) on sale of discontinued operations		886.8	0.0	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)	(0.0)
Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain (loss) on sale of discontinued operations		0.0	18.7	(2.3)
Held for Sale
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Restricted cash balance		0.0	89.2	78.4		$ 56.3
Held for Sale | NSM
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Short-term investments, at fair value			7.8
Cash held for sale			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		176.9	258.0	232.5
Other revenues		48.1	72.4	52.6
Total revenues - NSM Group		225.0	330.4	285.1
General and administrative expenses		126.8	190.4	179.5
Broker commission expenses		52.9	80.2	75.3
Change in fair value of contingent consideration		0.1	1.0	(3.3)
Amortization of other intangible assets		9.1	35.2	26.7
Loss on assets held for sale		0.0	(28.7)	0.0
Interest expense		12.1	23.3	22.1
Total expenses - NSM Group		201.0	358.8	300.3
Pre-tax income (loss) from discontinued operations - NSM Group		24.0	(28.4)	(15.2)
Income tax (expense) benefit		(7.6)	5.8	5.7
Net income (loss) from discontinued operations, net of tax - NSM Group		16.4	(22.6)	(9.5)
Gain (loss) on sale of discontinued operations		886.8	0.0	0.0
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		903.2	(3.9)	(11.8)
Net (income) loss from discontinued operations attributable to non-controlling interests		(0.7)	1.0	0.2
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders		902.5	(2.9)	(11.6)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group		(5.2)	0.2	5.9
Comprehensive income (loss) from discontinued operations		900.2	(2.7)	(5.7)
Other comprehensive income (loss) from discontinued operations attributable to non-controlling interests		0.2	(0.1)	(0.3)
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders		900.4	(2.8)	(6.0)
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations		38.7	42.3	35.5
Net cash provided from (used for) investing activities		7.1	(56.5)	(124.9)
Net cash used from (used for) financing activities		(17.5)	(1.0)	128.8
Effect of exchange rate changes on cash		4.0	0.2	(2.8)
Net decrease in cash during the year		32.3	(15.0)	36.6
Cash balance at beginning of year		111.6	126.6	90.0
Cash sold as part of the sale of NSM Group (includes restricted cash of $105.1 and $0.0)		(143.9)	0.0	0.0
Cash balance at end of year		0.0	111.6	126.6
Interest paid		(12.0)	(16.6)	(20.9)
Net income tax payments to US		0.0	0.0	0.0
Held for Sale | Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain (loss) on sale of discontinued operations		0.0	18.7	(2.3)
Discontinued Operations, Disposed of by Sale | NSM
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Loss on assets held for sale	$ (875.7)
Gain (loss) on sale of discontinued operations	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	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]
Gain (loss) on sale of discontinued operations			$ 17.6		$ 17.3