CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenues:
Earned insurance premiums	$ 281.3	$ 284.9	$ 838.1	$ 896.0
Net investment income	22.5	16.8	59.6	42.6
Net realized and unrealized investment gains (losses)	26.4	(43.9)	84.0	(33.9)
Other revenue	34.8	38.3	110.8	114.4
Total revenues	365.0	296.1	1,092.5	1,019.1
Expenses:
Loss and loss adjustment expenses	165.0	170.0	508.1	534.1
Insurance and Reinsurance Acquisition Expenses	56.4	55.3	159.2	166.1
Other Underwriting Expense	49.6	56.5	156.0	165.5
General and administrative expenses	70.2	97.3	230.4	231.6
Interest expense	3.8	3.9	12.4	10.8
Total expenses	345.0	383.0	1,066.1	1,108.1
Pre-tax income (loss) from continuing operations	20.0	(86.9)	26.4	(89.0)
Income tax benefit (expense)	6.7	1.6	22.4	(0.8)
Net income (loss) from continuing operations	26.7	(85.3)	48.8	(89.8)
Net gain on sale of discontinued operations			414.5	18.2
Net income (loss) from discontinued operations, net of tax	14.0	(3.8)	7.1	59.1
Income (loss) before equity in earnings of unconsolidated affiliates	88.6	(78.8)	470.4	(12.5)
Equity in earnings of unconsolidated affiliates, net of tax	0.0	3.9	0.0	18.0
Net income	88.6	(74.9)	470.4	5.5
Net loss (income) attributable to non-controlling interests	3.1	16.0	(24.6)	24.2
Net income (loss) attributable to White Mountains’s common shareholders	91.7	(58.9)	445.8	29.7
Other comprehensive income (loss), net of tax:
Change in equity in net unrealized gains (losses) from investments in Symetra common shares, net of tax	0.0	3.5	0.0	(29.4)
Change in foreign currency translation, pension liability and other, net of tax	0.2	0.1	0.3	0.3
Comprehensive income (loss)	91.9	(73.8)	591.4	(61.5)
Other comprehensive income (loss) attributable to non-controlling interests	0.1	(0.1)	0.1	(0.1)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 92.0	$ (73.9)	$ 591.5	$ (61.6)
Basic income (loss) per share
Continuing operations	$ 6.12	$ (11.10)	$ 4.67	$ (8.01)
Discontinued operations	12.72	1.09	81.60	12.98
Total consolidated operations	18.84	(10.01)	86.27	4.97
Diluted income (loss) per share
Continuing operations	6.11	(11.10)	4.66	(8.01)
Discontinued operations	12.69	1.09	81.47	12.98
Total consolidated operations	18.80	(10.01)	86.13	4.97
Dividends declared per White Mountains’s common share	$ 0.00	$ 0.00	$ 1.00	$ 1.00
Sale of Tranzact [Member]
Expenses:
Net gain on sale of discontinued operations	$ 51.9	$ 0.0	$ 51.9	$ 0.0
Sirius Group
Revenues:
Net realized and unrealized investment gains (losses)		0.8	3.7	3.5
Expenses:
Net gain on sale of discontinued operations	(4.0)	0.0	362.6	0.0
Other comprehensive income (loss), net of tax:
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease	0.0	(18.5)	32.0	(62.1)
Esurance
Expenses:
Net gain on sale of discontinued operations		10.3		18.2
Sale of Sirius Group [Member]
Other comprehensive income (loss), net of tax:
Accumulated Other Comprehensive Income Loss Net Of Tax Other Period Increase Decrease	$ 0.0	$ 0.0	$ 113.3	$ 0.0