CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 115.4	$ 20.5	$ (13.4)	$ 304.9
Other comprehensive income (loss), net of tax	0.5	(3.1)	(2.9)	(3.4)
Comprehensive income (loss)	115.9	17.4	(16.3)	301.5
Other Comprehensive (income) loss attributable to non-controlling interests	(0.2)	0.2	(0.2)	0.2
Comprehensive income (loss) attributable to White Mountains’s common shareholders	$ 115.7	$ 17.6	$ (16.5)	$ 301.7
Basic earnings (loss) per share
Continuing operations (in usd per share)	$ 37.46	$ 6.44	$ (4.25)	$ 95.71
Discontinued operations (in usd per share)	(0.32)	0	(0.03)	0.22
Total consolidated operations (in usd per share)	37.14	6.44	(4.28)	95.93
Diluted earnings (loss) per share
Continuing operations (in usd per share)	37.46	6.44	(4.25)	95.71
Discontinued operations (in usd per share)	(0.32)	0	(0.03)	0.22
Total consolidated operations (in usd per share)	37.14	6.44	(4.28)	95.93
Dividends declared per White Mountains’s common share	$ 0	$ 0	$ 1.00	$ 1.00