CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 284.4	$ (48.0)
Other comprehensive loss, net of tax	(0.3)	0.0
Comprehensive income	284.1	(48.0)
Comprehensive income (loss) attributable to non-controlling interests	0.0	0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 284.1	$ (48.0)
Basic earnings (loss) per share
Continuing operations (in usd per share)	$ 89.42	$ (12.85)
Discontinued operations (in usd per share)	0.22	0.03
Total consolidated operations (in usd per share)	89.64	(12.82)
Diluted earnings (loss) per share
Continuing operations (in usd per share)	89.42	(12.85)
Discontinued operations (in usd per share)	0.22	0.03
Total consolidated operations (in usd per share)	89.64	(12.82)
Dividends declared per White Mountains’s common share	$ 1.00	$ 1.00