CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net (loss) income attributable to White Mountains’s common shareholders	$ 3.5	$ 13.6	$ (44.5)	$ 42.6
Other comprehensive income, net of tax:
Other comprehensive (loss) income, net of tax	(0.9)	0.0	(0.9)	0.1
Comprehensive income from discontinued operations, net of tax	0.0	0.2	0.0	0.3
Comprehensive income (loss)	2.6	13.8	(45.4)	43.0
Other comprehensive income attributable to non-controlling interests	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 2.6	$ 13.8	$ (45.4)	$ 43.0
Basic earnings (loss) per share
Continuing operations (in usd per share)	$ 1.02	$ 2.36	$ (12.44)	$ 1.84
Discontinued operations (in usd per share)	0.00	0.61	0.03	7.47
Total consolidated operations (in usd per share)	1.02	2.97	(12.41)	9.31
Diluted earnings (loss) per share
Continuing operations (in usd per share)	1.02	2.36	(12.44)	1.84
Discontinued operations (in usd per share)	0.00	0.61	0.03	7.47
Total consolidated operations (in usd per share)	1.02	2.97	(12.41)	9.31
Dividends declared per White Mountains’s common share	$ 0.00	$ 0.00	$ 1.00	$ 1.00