Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Revenues (including realized gains and losses)	$ 626.6	$ 574.1	$ 489.3	$ 627.4	$ 566.4	$ 697.4	$ 542.3	$ 629.7	$ 2,317.4	$ 2,435.7	$ 2,173.1
Total expenses	492.4	518.8	485.7	475.6	617.2	562.3	504.7	488.8	1,972.5	2,172.9	2,075.1
Pre-tax loss	134.2	55.3	3.6	151.8	(50.8)	135.1	37.6	140.9	344.9	262.8	98.0
Tax benefit (expense)	(27.4)	(8.2)	0.6	(41.6)	101.0	(47.8)	(6.4)	(31.1)	(76.6)	15.7	110.0
Net income from continuing operations	106.8	47.1	4.2	110.2	50.2	87.3	31.2	109.8	268.3	278.5	208.0
Net loss from discontinued operations, net of tax	(0.3)	0.4	3.9	0.5	0.5	(106.8)	0.5	(9.2)	(42.1)	(24.0)	(36.7)
Non-controlling interest in consolidated subsidiaries	(0.3)	1.1	11.1	0.5	12.0	30.9	(12.1)	(16.8)	12.4	14.0	(41.5)
Equity in earnings (losses) of unconsolidated affiliates, net of tax	11.7	8.6	7.1	9.2	5.5	7.7	6.5	10.2	36.6	29.9	(20.2)
Net income attributable to White Mountains’ common shareholders	$ 117.9	$ 57.2	$ 26.3	$ 120.4	$ 68.2	$ 19.1	$ 26.1	$ 94.0	$ 321.8	$ 207.4	$ 767.9
Basic earnings (loss) per share
Continuing operations	$ 19.14	$ 9.20	$ 3.62	$ 19.10	$ 10.36	$ 19.11	$ 3.85	$ 13.85	$ 51.15	$ 47.41	$ 18.56
Discontinued operations	$ (0.05)	$ 0.06	$ 0.64	$ 0.07	$ 0.07	$ (16.21)	$ 0.07	$ (1.24)	$ 0.74	$ (16.91)	$ 78.88
Total consolidated operations	$ 19.09	$ 9.26	$ 4.26	$ 19.17	$ 10.43	$ 2.90	$ 3.92	$ 12.61	$ 51.89	$ 30.50	$ 97.44
Diluted earnings (loss) per share
Continuing operations	$ 19.14	$ 9.20	$ 3.62	$ 19.10	$ 10.36	$ 19.11	$ 3.85	$ 13.85	$ 51.15	$ 47.41	$ 18.56
Discontinued operations	$ (0.05)	$ 0.06	$ 0.64	$ 0.07	$ 0.07	$ (16.21)	$ 0.07	$ (1.24)	$ 0.74	$ (16.91)	$ 78.88
Total consolidated operations	$ 19.09	$ 9.26	$ 4.26	$ 19.17	$ 10.43	$ 2.90	$ 3.92	$ 12.61	$ 51.89	$ 30.50	$ 97.44
Adjusted book value per share	$ 642.27	$ 621.56	$ 604.75	$ 606.20	$ 587.63	$ 573.66	$ 564.77	$ 565.38