CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Gain (loss) from sale of other discontinued operations, net of tax	$ 415.1	$ 18.2	$ (1.6)
Revenues
Earned insurance premiums	1,114.0	1,188.2	1,185.0
Net investment income	86.8	60.8	59.5
Net realized and unrealized investment gains	10.3	225.4	78.5
Other revenue	149.6	147.3	88.1
Total revenues	1,360.7	1,621.7	1,411.1
Expenses
Loss and loss adjustment expenses	664.0	708.9	824.0
Insurance acquisition expenses	211.6	220.1	206.2
Other underwriting expenses	209.5	218.6	179.6
General and administrative expenses	299.9	302.2	216.5
Interest expense on debt	16.1	14.6	14.2
Total expenses	1,401.1	1,464.4	1,440.5
Pre-tax (loss) income from continuing operations	(40.4)	157.3	(29.4)
Income Tax Expense (Benefit)	45.4	0.2	14.8
Net income (loss) from continuing operations	5.0	157.5	(14.6)
Net (loss) income from discontinued operations, net of tax	(0.3)	78.7	260.6
Income before equity in earnings of unconsolidated affiliates	419.8	254.4	244.4
Equity in earnings of unconsolidated affiliates, net of tax	0.0	25.1	45.6
Net income	419.8	279.5	290.0
Net (income) loss attributable to non-controlling interests	(7.3)	18.1	22.2
Net income attributable to White Mountains’s common shareholders	412.5	297.6	312.2
Other comprehensive income, net of tax:
Change in equity in net unrealized investment (losses) gains in Symetra common shares, net of tax	0.0	(34.9)	75.3
OCI, Change in foreign currency transaction and translation adjustment, net of tax	0.3	(0.5)	(10.7)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax		0.0	0.0
Comprehensive income	558.1	197.2	207.3
Comprehensive (income) loss attributable to non-controlling interests	(0.3)	0.0	3.3
Comprehensive income attributable to White Mountains’s common shareholders	$ 557.8	$ 197.2	$ 210.6
Basic earnings per share
Continuing operations	$ (0.47)	$ 34.12	$ 8.70
Discontinued operations	82.71	16.48	42.43
Total consolidated operations	82.24	50.60	51.13
Diluted earnings per share
Continuing operations	(0.47)	34.12	8.70
Discontinued operations	82.66	16.48	42.43
Total consolidated operations	82.19	50.60	51.13
Dividends declared and paid per White Mountains’s common share	$ 1.00	$ 1.00	$ 1.00
Sirius Group
Gain (loss) from sale of other discontinued operations, net of tax	$ 363.2
Revenues
Net realized and unrealized investment gains	3.7	$ 205.0	$ 2.7
Tranzact
Gain (loss) from sale of other discontinued operations, net of tax	51.9
Esurance [Member]
Gain (loss) from sale of other discontinued operations, net of tax	0.0	17.9	3.2
Other Discontinued Operations [Member]
Gain (loss) from sale of other discontinued operations, net of tax	0.0	0.3	(4.8)
Sale of Sirius Group [Member]
Other comprehensive income, net of tax:
OCI, Change in foreign currency transaction and translation adjustment, net of tax	32.0	$ (65.0)	$ (169.5)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax	$ 113.3