Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 520.8	$ 459.5	$ 259.6	$ (59.2)	$ 175.3	$ 155.5	$ 129.0	$ 433.6	$ 1,180.7	$ 893.4	$ 369.1
Expenses	163.5	138.3	126.9	106.8	118.8	104.4	113.2	151.9	535.5	488.3	547.3
Pre-tax income (loss)	357.3	321.2	132.7	(166.0)	56.5	51.1	15.8	281.7	645.2	405.1	(178.2)
Tax (expense) benefit	117.6	(98.5)	(24.1)	25.5	(10.4)	(8.8)	0.1	(10.2)	20.5	(29.3)	4.0
Net income (loss) from continuing operations	474.9	222.7	108.6	(140.5)	46.1	42.3	15.9	271.5	665.7	375.8	(174.2)
Net income from discontinued operations, net of tax	(1.5)	(0.7)	(1.0)	0.9	(0.8)	0.9	0.0	0.7
Non-controlling interest in consolidated subsidiaries	15.8	10.9	7.8	10.8	15.6	5.5	4.6	12.2	45.3	37.9	50.2
Net income (loss) attributable to White Mountains’s common shareholders	$ 489.2	$ 232.9	$ 115.4	$ (128.8)	$ 60.9	$ 48.7	$ 20.5	$ 284.4	$ 708.7	$ 414.5	$ (141.2)
Basic earnings (loss) per share
Continuing operations (usd per share)	$ 158.19	$ 75.32	$ 37.46	$ (40.82)	$ 19.37	$ 15.01	$ 6.44	$ 89.42	$ 227.72	$ 130.02	$ (36.67)
Discontinued operations (usd per share)	(0.49)	(0.23)	(0.32)	0.28	(0.25)	0.28	0	0.22	(0.75)	0.25	(5.09)
Total consolidated operations (usd per share)	157.70	75.09	37.14	(40.54)	19.12	15.29	6.44	89.64	226.97	130.27	(41.76)
Diluted earnings (loss) per share
Continuing operations (usd per share)	158.19	75.32	37.46	(40.82)	19.37	15.01	6.44	89.42	227.72	130.02	(36.67)
Discontinued operations (usd per share)	(0.49)	(0.23)	(0.32)	0.28	(0.25)	0.28	0	0.22	(0.75)	0.25	(5.09)
Total consolidated operations (usd per share)	$ 157.70	$ 75.09	$ 37.14	$ (40.54)	$ 19.12	$ 15.29	$ 6.44	$ 89.64	$ 226.97	$ 130.27	$ (41.76)