Discontinued Operations - (Details) - USD ($)					3 Months Ended				6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Jul. 21, 2016	Apr. 18, 2016	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold										$ 1,131,000,000	$ 2,657,200,000
Gain from sale of other discontinued operations, net of tax					$ 554,300,000		$ 47,900,000			552,700,000	414,500,000
Net (loss) income from discontinued operations					(15,200,000)		36,500,000			20,500,000	100,900,000
Income tax expense (benefit)					(4,000,000)		(17,100,000)			(5,300,000)	(22,700,000)
Payments to acquire other investments										67,700,000	36,600,000
Investments					3,280,500,000					3,280,500,000		$ 2,714,400,000
Net realized and unrealized investment gains					32,500,000		10,900,000			102,500,000	27,200,000
Net income attributable to White Mountains’s common shareholders					(539,100,000)		(84,400,000)			(573,200,000)	(515,400,000)
Net change in foreign currency translation and benefit plan assets and obligations					0		200,000			200,000	(400,000)
OneBeacon
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold	$ 1,300,000,000
Gain from sale of other discontinued operations, net of tax	$ 554,600,000
Net (loss) income from discontinued operations					(15,200,000)		22,500,000			20,500,000	93,800,000
Tranzact
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1,200,000	$ 221,300,000
Gain from sale of other discontinued operations, net of tax		51,900,000				$ (1,000,000)						51,900,000
Net (loss) income from discontinued operations							14,000,000				13,500,000
Discontinued operation, tax effect of discontinued operation		$ 30,200,000					15,900,000				13,800,000	30,200,000
Net realized and unrealized investment gains							0				0
Sirius Group
Discontinued Operations [Line Items}
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600,000,000
Gain from sale of other discontinued operations, net of tax							(4,000,000)					363,200,000
Payments to acquire other investments				$ 161,800,000
Investments													$ 686,200,000
Net realized and unrealized investment gains								$ 400,000	$ 3,700,000
Sale of Sirius Group
Discontinued Operations [Line Items}
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary												113,300,000
Discontinued Operations
Discontinued Operations [Line Items}
Net income attributable to White Mountains’s common shareholders					539,100,000		84,400,000			573,200,000	515,400,000
Net change in foreign currency translation and benefit plan assets and obligations											32,500,000
Discontinued Operations | OneBeacon
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations					(15,200,000)		22,500,000			20,500,000	93,800,000
Discontinued operation, tax effect of discontinued operation					4,100,000		(10,400,000)			5,700,000	(300,000)
Net realized and unrealized investment gains					11,500,000		15,500,000			38,800,000	56,800,000
Net income attributable to White Mountains’s common shareholders					539,300,000		22,500,000			575,000,000	93,800,000
Net change in foreign currency translation and benefit plan assets and obligations					100,000		(300,000)			300,000	500,000
Discontinued Operations | Tranzact
Discontinued Operations [Line Items}
Net (loss) income from discontinued operations							2,100,000				2,600,000
Discontinued operation, tax effect of discontinued operation												30.2
Income tax expense (benefit)												$ 16.1
Net income attributable to White Mountains’s common shareholders							65,900,000			(1,000,000)	65,400,000
Discontinued Operations | Sirius Group
Discontinued Operations [Line Items}
Gain from sale of other discontinued operations, net of tax					(800,000)					1,000,000
Net (loss) income from discontinued operations					0		0				(6,400,000)
Discontinued operation, tax effect of discontinued operation					0		0				3,100,000
Net realized and unrealized investment gains					0		0				(1,500,000)
Net income attributable to White Mountains’s common shareholders					$ (200,000)		(4,000,000)			$ (800,000)	356,200,000
Other comprehensive income											145,300,000
Net change in foreign currency translation and benefit plan assets and obligations							0				32,000,000
Discontinued Operations | Sale of Sirius Group
Discontinued Operations [Line Items}
Net change in foreign currency translation and benefit plan assets and obligations							0				113,300,000
Continuing Operations
Discontinued Operations [Line Items}
Income tax expense (benefit)											$ (14,000,000)
Continuing Operations | Tranzact
Discontinued Operations [Line Items}
Income tax expense (benefit)							$ 14,000,000
Symetra | Sirius Group
Discontinued Operations [Line Items}
Investments													$ 528,600,000
Intact Financial Corporation [Member] | OneBeacon
Discontinued Operations [Line Items}
Sale of Stock, Price Per Share	$ 18.10