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SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 2) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Fair value investments	$ 5,039.7				$ 4,050.9				$ 5,039.7	$ 4,050.9	$ 3,343.6
(Loss) revenues (including realized gains and losses)	268.2	$ 365.0	$ 358.2	$ 369.3	602.6	$ 296.1	$ 368.7	$ 354.3	1,360.7	1,621.7	1,411.1
Expenses	335.0	345.0	360.3	360.8	356.3	383.0	373.8	351.3	1,401.1	1,464.4	1,440.5
Pre-tax loss	(66.8)	20.0	(2.1)	8.5	246.3	(86.9)	(5.1)	3.0	(40.4)	157.3	(29.4)
Income Tax Expense (Benefit)	23.0	6.7	6.0	9.7	1.0	1.6	2.2	(4.6)	45.4	0.2	14.8
Net income (loss) from continuing operations	(43.8)	26.7	3.9	18.2	247.3	(85.3)	(2.9)	(1.6)	5.0	157.5	(14.6)
Net (loss) income from discontinued operations, net of tax	(6.8)	61.9	358.6	1.1	19.6	6.5	(0.5)	71.3	(0.3)	78.7	260.6
Net income attributable to White Mountains’s common shareholders	(33.3)	91.7	341.1	13.0	267.9	(58.9)	4.3	84.3	412.5	297.6	312.2
Comprehensive income attributable to White Mountains’ common shareholders									557.8	197.2	210.6
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders	(33.3)	$ 91.7	$ 341.1	$ 13.0	267.9	$ (58.9)	$ 4.3	$ 84.3	412.5	297.6	312.2
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
(Loss) revenues (including realized gains and losses)									(1.0)	5.0	0.3
Expenses									68.2	59.9	32.3
Pre-tax loss									(69.2)	(54.9)	(32.0)
Income Tax Expense (Benefit)									(0.5)	0.0	0.9
Net income (loss) from continuing operations									(69.7)	(54.9)	(31.1)
Net (loss) income from discontinued operations, net of tax									0.0	0.0	13.9
Equity in earnings (losses) from consolidated and unconsolidated affiliates									482.2	352.5	329.4
Net income attributable to White Mountains’s common shareholders									412.5	297.6	312.2
Other comprehensive income (loss), after-tax									145.3	(100.4)	(101.6)
Comprehensive income attributable to White Mountains’ common shareholders									557.8	197.2	210.6
Computation of net income (loss) available to common shareholders:
Net income attributable to White Mountains’s common shareholders									412.5	297.6	$ 312.2
Fair value measured on a recurring basis
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Fair value investments	5,195.9				4,134.4				5,195.9	4,134.4
Level 3 | Fair value measured on a recurring basis
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Fair value investments	177.7				$ 239.5				177.7	$ 239.5
Level 3 | Forward Contracts [Member] | Fair value measured on a recurring basis
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Fair value investments	$ (1.2)								$ (1.2)

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The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.

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