CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income attributable to White Mountains’s common shareholders	$ (141.2)	$ 627.2	$ 401.8
Other comprehensive income, net of tax:
Other comprehensive (loss) income, net of tax	(4.8)	0.3	(0.7)
Comprehensive income from discontinued operations, net of tax	0.0	3.2	146.3
Comprehensive income	(146.0)	630.7	547.4
Comprehensive loss (income) attributable to non-controlling interests	0.3	(0.2)	(0.3)
Comprehensive income attributable to White Mountains’s common shareholders	$ (145.7)	$ 630.5	$ 547.1
Basic earnings per share
Continuing operations	$ (36.67)	$ 11.56	$ (24.26)
Discontinued operations	(5.09)	134.50	104.37
Total consolidated operations	(41.76)	146.06	80.11
Diluted earnings per share
Continuing operations	(36.67)	11.56	(24.26)
Discontinued operations	(5.09)	134.50	104.32
Total consolidated operations	(41.76)	146.06	80.06
Dividends declared and paid per White Mountains’s common share	$ 1.00	$ 1.00	$ 1.00