Financial Statement Revisions - Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets
Goodwill	$ 25.9	$ 25.9	$ 25.9	$ 25.9
Total assets	3,587.0	3,659.2	6,490.4
Liabilities
Total liabilities	291.2	298.4	2,759.6
White Mountains’s common shareholders’ equity
White Mountains’ common shares	3.8	3.8	4.6
Paid-in surplus	671.5	666.8	803.2
Retained earnings	2,765.0	2,823.2	2,795.9
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations			(4.4)
Total White Mountains’s common shareholders’ equity	3,439.0	3,492.5	3,599.3
Non-controlling interests	(143.2)	(131.7)	131.5
Total equity	3,295.8	3,360.8	3,730.8
Total liabilities and equity	3,587.0	3,659.2	6,490.4
As previously reported
Assets
Total assets			6,516.8
Liabilities
Total liabilities			2,759.3
White Mountains’s common shareholders’ equity
White Mountains’ common shares			4.6
Paid-in surplus			802.5
Retained earnings			2,821.7
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations			(3.7)
Total White Mountains’s common shareholders’ equity			3,625.1
Non-controlling interests			132.4
Total equity			3,757.5
Total liabilities and equity			6,516.8
Adjustments
Assets
Total assets			(26.4)
Liabilities
Total liabilities			0.3
White Mountains’s common shareholders’ equity
White Mountains’ common shares			0.0
Paid-in surplus			0.7
Retained earnings			(25.8)
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations			(0.7)
Total White Mountains’s common shareholders’ equity			(25.8)
Non-controlling interests			(0.9)
Total equity			(26.7)
Total liabilities and equity			(26.4)
HG Global-BAM
Assets
Other assets	5.1	5.6
Total assets	750.1	747.4	699.1
Liabilities
Other liabilities	12.2	11.4
Total liabilities	179.6	167.0	126.4
HG Global-BAM | As previously reported
Assets
Total assets			699.1
Liabilities
Total liabilities			126.4
HG Global-BAM | Adjustments
Assets
Total assets			0.0
Liabilities
Total liabilities			0.0
MediaAlpha
Assets
Other assets	1.6	1.3
Total assets	103.7	96.5	53.2
Liabilities
Other liabilities	0.9	2.4
Total liabilities	60.7	59.8	25.6
Accumulated other comprehensive loss, after tax:
Non-controlling interests	16.2	13.1
MediaAlpha | As previously reported
Assets
Total assets			53.2
Liabilities
Total liabilities			25.6
MediaAlpha | Adjustments
Assets
Total assets			0.0
Liabilities
Total liabilities			0.0
Other Entity
Assets
Goodwill			7.6
Other intangible assets			1.1
Other assets — commissions receivable			4.2
Assets held for sale	3.3	3.3	3,578.9
Other assets	16.5	19.1	2,146.3
Total assets	2,733.2	2,815.3
Liabilities
Other liabilities			2,535.2
Other liabilities	9.5	11.0	72.4
Total liabilities	$ 50.9	$ 71.6
Other Entity | As previously reported
Assets
Goodwill			13.4
Other intangible assets			4.5
Other assets — commissions receivable			21.4
Assets held for sale			3,578.9
Other assets			2,146.3
Liabilities
Other liabilities			2,535.2
Other liabilities			72.1
Other Entity | Adjustments
Assets
Goodwill			(5.8)
Other intangible assets			(3.4)
Other assets — commissions receivable			(17.2)
Assets held for sale			0.0
Other assets			0.0
Liabilities
Other liabilities			0.0
Other liabilities			$ 0.3