Held for Sale and Discontinued Operations - Financial Statement Disclosure (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021	Dec. 31, 2020
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net income (loss) income from NSM Group discontinued operations	$ 6.4	$ 1.1	$ 10.1	$ (29.1)
Gain (loss) from sale of Sirius Group, net of tax	0.0	0.0	0.0	18.7
Total income (loss) from discontinued operations, net of tax	(6.4)	(1.1)	(10.1)	10.4
Net (income) loss from discontinued operations attributable to non-controlling interests	(0.5)	0.0	(0.6)	1.1
Other comprehensive income (loss) from NSM Group discontinued operations, net of tax	(4.0)	0.5	(5.9)	2.6
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations			36.7	42.4
Net cash provided from (used for) investing activities			7.1	(2.6)
Net cash used from (used for) financing activities			(16.9)	(11.2)
Net change in cash during the period			235.7	87.3
Cash balances at beginning of period (includes restricted cash balances of $4.5 and $— 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 $7.3 and $4.0 and excludes discontinued operations cash balances of $142.7 and 154.8)	383.4	171.9	383.4	171.9
Restricted cash balance	7.3	4.0	7.3	4.0	$ 4.5	$ 0.0
Interest paid			6.3	3.4
Net income tax payments			3.8	2.1
Discontinued Operations, Held-for-sale | NSM Insurance Group
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Short-term investments, at fair value	0.0		0.0		7.8
Cash (restricted $108.9 and $89.2)	142.7		142.7		111.6
Restricted cash	108.9		108.9		89.2
Premiums and commissions receivable	97.8		97.8		85.0
Goodwill and other intangible assets	698.1		698.1		725.4
Other assets	59.1		59.1		59.2
Total assets held for sale	997.7		997.7		989.0
Debt	261.7		261.7		272.1
Premiums payable	163.4		163.4		135.9
Contingent consideration	0.0		0.0		6.8
Other liabilities	79.3		79.3		80.5
Total liabilities held for sale	504.4		504.4		495.3
Net assets held for sale	493.3		493.3		493.7
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Commission revenues	80.2	68.0	150.3	127.6
Other revenues	22.7	16.3	41.1	31.5
Total revenues - NSM Group	102.9	84.3	191.4	159.1
General and administrative expenses	57.1	47.7	111.1	93.5
Broker commission expenses	24.1	21.6	44.7	40.5
Change in fair value of contingent consideration	0.0	0.0	0.1	0.2
Amortization of other intangible assets	0.0	8.2	9.1	16.8
Loss on assets held for sale	0.0	0.0	0.0	(28.7)
Interest expense	8.4	5.9	10.5	11.8
Total expenses - NSM Group	89.6	83.4	175.5	191.5
Pre-tax income (loss) from NSM Group discontinued operations	13.3	0.9	15.9	(32.4)
Income tax (expense) benefit	(6.9)	0.2	(5.8)	3.3
Net income (loss) income from NSM Group discontinued operations	6.4	1.1	10.1	(29.1)
Gain (loss) from sale of Sirius Group, net of tax	0.0	0.0	0.0	18.7
Total income (loss) from discontinued operations, net of tax	6.4	1.1	10.1	(10.4)
Net (income) loss from discontinued operations attributable to non-controlling interests	(0.5)	0.0	(0.6)	1.1
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders	5.9	1.1	9.5	(9.3)
Other comprehensive income (loss) from NSM Group discontinued operations, net of tax	(4.0)	0.5	(5.9)	2.6
Comprehensive income (loss) from discontinued operations	1.9	1.6	3.6	(6.7)
Other comprehensive income (loss) from NSM discontinued operations attributable to non-controlling interests	0.2	0.0	0.3	(0.1)
Comprehensive income (loss) from discontinued operations attributable to White Mountains’s common shareholders	2.1	1.6	3.9	(6.8)
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from (used for) operations			36.7	42.4
Net cash provided from (used for) investing activities			7.1	(2.6)
Net cash used from (used for) financing activities			(16.9)	(11.2)
Effect of exchange rate changes on cash			4.2	(0.4)
Net change in cash during the period			31.1	28.2
Cash balances at beginning of period (includes restricted cash balances of $4.5 and $— and excludes discontinued operations cash balances of $111.6 and $126.6)			111.6	126.6
Cash balances at end of period (includes restricted cash balances of $7.3 and $4.0 and excludes discontinued operations cash balances of $142.7 and 154.8)	142.7	154.8	142.7	154.8
Restricted cash balance	$ 108.9	$ 90.3	108.9	90.3	$ 89.2	$ 78.4
Interest paid			10.4	11.1
Net income tax payments			$ 0.0	$ 0.0