Financial Statement Revisions - Statement of Operations and Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	$ 373.8	$ 157.7	$ 440.0
Advertising & commission revenues									167.0	118.3	107.4
Total expenses		79.1	85.7	92.4	69.0	70.6	74.1	91.3	366.0	305.0	310.8
Amortization of other intangible assets									10.7	10.5	8.4
Pre-tax (loss) income from continuing operations	$ 5.2	8.4	(2.2)	(3.6)	(74.5)	(13.8)	(25.7)	(33.3)	7.8	(147.3)	129.2
Income tax benefit (expense)	2.5	4.0	1.0	0.3	10.2	17.1	4.0	1.6	7.8	32.9	(12.7)
Net income (loss) from continuing operations	7.7	12.4	(1.2)	(3.3)	(64.3)	3.3	(21.7)	(31.7)	15.6	(114.4)	116.5
Gain (loss) from sale of other discontinued operations, net of tax									557.0	415.1	18.2
Net (loss) income from discontinued operations, net of tax	4.3	539.1	2.8	31.3	8.0	84.4	383.6	47.4	20.5	108.3	116.9
Income before equity in earnings of unconsolidated affiliates									593.1	409.0	251.6
Equity in earnings of unconsolidated affiliates									0.0	0.0	25.1
Net income									593.1	409.0	276.7
Net (income) loss attributable to non-controlling interests	10.5	10.6	12.0	1.0	17.4	3.1	(21.5)	(6.2)	34.1	(7.2)	18.5
Net income attributable to White Mountains’s common shareholders (3)	$ 22.5	$ 562.1	13.6	29.0	(38.9)	$ 90.8	340.4	9.5	627.2	401.8	295.2
Other comprehensive income (loss), net of tax									0.3	(0.7)	(42.8)
Comprehensive income									630.7	547.4	187.4
Comprehensive (income) loss attributable to non-controlling interests									(0.2)	(0.3)	0.0
Comprehensive income attributable to White Mountains’s common shareholders									630.5	$ 547.1	$ 187.4
Basic and diluted earnings per share - continuing operations										$ (24.26)	$ 27.22
Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues			85.4	93.5	0.3		49.2	58.8		$ 166.3	$ 435.3
Total expenses			85.8	92.2	69.1		74.3	88.5		302.8	303.3
Pre-tax (loss) income from continuing operations			(0.4)	1.3	(68.8)		(25.1)	(29.7)		(136.5)	132.0
Income tax benefit (expense)			1.0	0.3	10.2		4.0	1.6		32.9	(12.7)
Net income (loss) from continuing operations			0.6	1.6	(58.6)		(21.1)	(28.1)		(103.6)	119.3
Gain (loss) from sale of other discontinued operations, net of tax										415.1	18.2
Net (loss) income from discontinued operations, net of tax			2.8	31.3	8.0		383.6	47.4		108.3	116.9
Income before equity in earnings of unconsolidated affiliates										419.8	254.4
Equity in earnings of unconsolidated affiliates										0.0	25.1
Net income										419.8	279.5
Net (income) loss attributable to non-controlling interests			12.1	1.3	17.3		(21.4)	(6.3)		(7.3)	18.1
Net income attributable to White Mountains’s common shareholders (3)			15.5	34.2	(33.3)		341.1	13.0		412.5	297.6
Comprehensive income										558.1	197.2
Comprehensive (income) loss attributable to non-controlling interests										(0.3)	0.0
Comprehensive income attributable to White Mountains’s common shareholders										$ 557.8	$ 197.2
Basic and diluted earnings per share - continuing operations										$ (22.13)	$ 27.63
Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues			(1.9)	(4.7)	(5.8)		(0.8)	(0.8)		$ (8.6)	$ 4.7
Total expenses			(0.1)	0.2	(0.1)		(0.2)	2.8		2.2	7.5
Pre-tax (loss) income from continuing operations			(1.8)	(4.9)	(5.7)		(0.6)	(3.6)		(10.8)	(2.8)
Income tax benefit (expense)				0.0	0.0		0.0	0.0		0.0	0.0
Net income (loss) from continuing operations			(1.8)	(4.9)	(5.7)		(0.6)	(3.6)		(10.8)	(2.8)
Gain (loss) from sale of other discontinued operations, net of tax										0.0	0.0
Net (loss) income from discontinued operations, net of tax			0.0	0.0	0.0		0.0	0.0		0.0	0.0
Income before equity in earnings of unconsolidated affiliates										(10.8)	(2.8)
Equity in earnings of unconsolidated affiliates										0.0	0.0
Net income										(10.8)	(2.8)
Net (income) loss attributable to non-controlling interests			(0.1)	(0.3)	0.1		(0.1)	0.1		0.1	0.4
Net income attributable to White Mountains’s common shareholders (3)			$ (1.9)	$ (5.2)	$ (5.6)		$ (0.7)	$ (3.5)		(10.7)	(2.4)
Comprehensive income										(10.7)	(9.8)
Comprehensive (income) loss attributable to non-controlling interests										0.0	0.0
Comprehensive income attributable to White Mountains’s common shareholders										$ (10.7)	$ (9.8)
Basic and diluted earnings per share - continuing operations										$ (2.13)	$ (0.41)
HG Global BAM
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues									23.3	$ 16.7	$ 10.7
Total expenses									47.3	43.4	40.1
HG Global BAM | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										16.7	10.7
Total expenses										43.4	40.1
HG Global BAM | Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										0.0	0.0
Total expenses										0.0	0.0
MediaAlpha
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues									163.2	116.5	105.5
Advertising & commission revenues									163.2	116.5	105.5
Total expenses									163.6	120.6	107.5
Amortization of other intangible assets									10.5	10.1	8.1
MediaAlpha | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										116.5	105.5
Total expenses										120.6	107.5
MediaAlpha | Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										0.0	0.0
Total expenses										0.0	0.0
Other
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues									187.3	24.5	323.8
Advertising & commission revenues									3.8	1.8	1.9
Other revenues										22.7	321.9
Total expenses									155.1	141.0	163.2
Other expense										140.6	162.7
Amortization of other intangible assets									0.2	0.4	0.5
Other | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										33.1	319.1
Advertising & commission revenues										10.4	4.6
Other revenues										22.7	314.5
Total expenses										138.8	155.7
Other expense										137.6	154.5
Amortization of other intangible assets										1.2	1.2
Other | Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total revenues										(8.6)	4.7
Advertising & commission revenues										(8.6)	(2.7)
Other revenues										0.0	7.4
Total expenses										2.2	7.5
Other expense										3.0	8.2
Amortization of other intangible assets										(0.8)	(0.7)
Discontinued Operations
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income attributable to White Mountains’s common shareholders (3)									$ 627.2	401.8	295.2
Other comprehensive income (loss), net of tax										145.6	(107.8)
Discontinued Operations | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other comprehensive income (loss), net of tax										145.6	(100.4)
Discontinued Operations | Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other comprehensive income (loss), net of tax										$ 0.0	$ (7.4)