CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income attributable to White Mountains’s common shareholders (3)	$ 627.2	$ 401.8	$ 295.2
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax	0.3	(0.7)	(42.8)
Comprehensive income (loss) from discontinued operations, net of tax	3.2	146.3	(65.0)
Comprehensive income	630.7	547.4	187.4
Comprehensive (income) loss attributable to non-controlling interests	(0.2)	(0.3)	0.0
Comprehensive income attributable to White Mountains’s common shareholders	$ 630.5	$ 547.1	$ 187.4
Basic earnings per share
Continuing operations	$ 11.56	$ (24.26)	$ 27.22
Discontinued operations	134.50	104.37	22.98
Total consolidated operations	146.06	80.11	50.20
Diluted earnings per share
Continuing operations	11.56	(24.26)	27.22
Discontinued operations	134.50	104.32	22.98
Total consolidated operations	146.06	80.06	50.20
Dividends declared and paid per White Mountains’s common share	$ 1	$ 1	$ 1