Discontinued Operations - (Details) $ / shares in Units, $ in Millions					3 Months Ended						6 Months Ended		12 Months Ended
	May 02, 2017 $ / shares	Oct. 05, 2016 USD ($)	Jul. 21, 2016 USD ($)	Apr. 18, 2016 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold											$ 0.0	$ 2,248.5
Net (loss) gain on sale of discontinued operations, net of tax							$ (0.6)			$ 366.6	(1.6)	366.6
Income tax expense (benefit)							(1.0)			(4.0)	(1.3)	(5.6)
Net (loss) income from discontinued operations							3.4			17.0	35.7	64.4
Payments to acquire other investments											55.1	12.9
Investments							2,692.9				2,692.9		$ 2,714.4
Net realized and unrealized investment gains							33.7			3.4	70.0	16.3
Net change in foreign currency translation and benefit plan assets and obligations							0.6			(0.2)	1.4	(0.1)
Tranzact
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1.2	$ 221.3
Net (loss) gain on sale of discontinued operations, net of tax		51.9						$ (1.0)					51.9
Discontinued operation, tax effect of discontinued operation		$ 30.2											30.2
Sirius Group
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600.0
Net (loss) gain on sale of discontinued operations, net of tax							(0.6)		$ 4.0				363.2
Payments to acquire other investments				$ 161.8
Investments														$ 686.2
Net realized and unrealized investment gains										0.4		3.7
Sale of Sirius Group
Discontinued Operations [Line Items}
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of Sirius Group													113.3
Discontinued Operations
Discontinued Operations [Line Items}
Discontinued operation, tax effect of discontinued operation							5.8			3.1	1.6	11.1
Net (loss) income from discontinued operations							3.4			17.0	35.7	64.4
Net realized and unrealized investment gains							12.3			32.0	27.3	39.8
Net income attributable to White Mountains’s common shareholders							2.8			383.6	34.1	431.0
Net change in foreign currency translation and benefit plan assets and obligations							0.2			108.3	0.3	145.5
Discontinued Operations | Tranzact
Discontinued Operations [Line Items}
Discontinued operation, tax effect of discontinued operation													8.8
Income tax expense (benefit)													(21.4)
Tax effect of extraordinary item, gain (loss)													$ 30.2
Net (loss) income from discontinued operations										(2.5)		(0.5)
Net income attributable to White Mountains’s common shareholders											(1.0)
Discontinued Operations | Sirius Group
Discontinued Operations [Line Items}
Net (loss) gain on sale of discontinued operations, net of tax							(0.6)
Discontinued operation, tax effect of discontinued operation							0.0			2.2		3.1
Net (loss) income from discontinued operations							0.0			(5.5)		(6.4)
Net realized and unrealized investment gains							0.0			7.3		(1.5)
Net income attributable to White Mountains’s common shareholders							$ (0.6)			361.1	$ (0.6)	360.2
Other comprehensive income										108.1		145.3
Net change in foreign currency translation and benefit plan assets and obligations										(5.2)		32.0
Discontinued Operations | Sale of Sirius Group
Discontinued Operations [Line Items}
Net change in foreign currency translation and benefit plan assets and obligations										$ 113.3		$ 113.3
Intact Financial Corporation | OneBeacon
Discontinued Operations [Line Items}
Sale of stock, price per share | $ / shares	$ 18.10
Transaction book value multiple	1.65
OneBeacon
Discontinued Operations [Line Items}
Ownership interest (as a percent)							75.70%				75.70%
Business acquisition, percentage of voting interests acquired							96.90%				96.90%
Scenario, Forecast
Discontinued Operations [Line Items}
Proceeds from acquisition					$ 1,300.0	$ 1,300.0
Symetra | Sirius Group
Discontinued Operations [Line Items}
Investments														$ 528.6