Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues (including realized gains and losses)	$ 678.5	$ 334.7	$ 403.6	$ 391.8	$ 405.6	$ 323.2	$ 393.5	$ 331.8	$ 1,808.6	$ 1,454.1	$ 1,361.6
Total expenses	420.4	429.3	412.7	391.3	471.3	358.7	359.9	293.2	1,653.7	1,483.1	1,203.0
Pre-tax loss	258.1	(94.6)	(9.1)	0.5	(65.7)	(35.5)	33.6	38.6	154.9	(29.0)	158.6
Tax benefit (expense)	1.5	1.6	2.2	(4.6)	26.2	4.7	(7.1)	(9.0)	0.7	14.8	(32.6)
Net income (loss) from continuing operations	259.6	(93.0)	(6.9)	(4.1)	(39.5)	(30.8)	26.5	29.6	155.6	(14.2)	126.0
Net income from discontinued operations, net of tax	7.3	14.2	3.5	73.8	82.1	63.9	61.1	51.5	80.6	260.2	99.9
Non-controlling interest in consolidated subsidiaries	(6.1)	16.0	0.9	7.3	15.0	11.2	(4.6)	0.6	18.1	22.2	12.5
Equity in earnings of unconsolidated affiliates, net of tax	7.1	3.9	6.8	7.3	12.3	7.0	12.5	13.8	25.1	45.6	36.6
Net income attributable to White Mountains’s common shareholders	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 69.9	$ 51.3	$ 95.5	$ 95.5	$ 297.6	$ 312.2	$ 321.6
Basic earnings per share
Continuing operations	$ 45.99	$ (12.42)	$ 0.13	$ 1.76	$ (2.03)	$ (2.07)	$ 5.58	$ 7.13	$ 33.80	$ 8.77	$ 28.22
Discontinued operations	1.29	2.41	0.59	12.33	13.68	10.49	9.92	8.35	16.80	42.36	23.63
Total consolidated operations	47.28	(10.01)	0.72	14.09	11.65	8.42	15.50	15.48	50.60	51.13	51.85
Diluted earnings per share
Continuing operations	45.96	(12.42)	0.13	1.76	(2.03)	(2.07)	5.58	7.13	33.80	8.77	28.22
Discontinued operations	1.29	2.41	0.59	12.33	13.68	10.49	9.92	8.35	16.80	42.36	23.63
Total consolidated operations	$ 47.25	$ (10.01)	$ 0.72	$ 14.09	$ 11.65	$ 8.42	$ 15.50	$ 15.48	$ 50.60	$ 51.13	$ 51.85