SELECTED QUARTERLY FINANCIAL DATA (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
SELECTED QUARTERLY FINANCIAL DATA (Unaudited)
Revenues	$ 593.6	$ 501.3	$ 553.6	$ 529.6	$ 552.8	$ 551.0	$ 670.8	$ 792.1	$ 2,178.1	$ 2,566.7	$ 3,443.3
Expenses	535.6	500.1	517.5	563.3	457.2	475.8	664.9	815.7	2,116.5	2,413.6	2,646.1
Pre-tax income	58.0	1.2	36.1	(33.7)	95.6	75.2	5.9	(23.6)	61.6	153.1	797.2
Tax benefit (expense)	124.6	0.6	(10.0)	7.7	(6.6)	(11.7)	1.4		122.7	(16.9)	(214.2)
Net income from continuing operations	182.6	1.8	26.1	(26.0)	89.0	63.5	7.3	(23.6)	184.3	136.2	583.0
Net loss from discontinued operations, net of tax	674.2	(30.1)	(1.5)	2.5	(7.8)	3.6	0.6	(3.0)	(13.0)	(6.6)	(27.8)
Noncontrolling interest in consolidated subsidiaries	(20.3)	11.0	(20.8)	(11.4)	(16.6)	(24.3)	(10.7)	(1.4)	(41.5)	(53.0)	(109.5)
Equity in (losses) earnings of unconsolidated affiliates	(36.3)	1.5	7.9	6.7	8.3	7.9	5.3	(11.6)	(20.2)	9.9	24.3
Net income attributable to White Mountains' common shareholders	$ 800.2	$ (15.8)	$ 11.7	$ (28.2)	$ 72.9	$ 50.7	$ 2.5	$ (39.6)	$ 767.9	$ 86.5	$ 470.0
Basic
Continuing operations (in dollars per share)	$ 16.54	$ 1.81	$ 1.66	$ (3.82)	$ 9.77	$ 5.58	$ 0.21	$ (4.14)	$ 15.56	$ 10.89	$ 56.25
Discontinued operations (in dollars per share)	$ 88.50	$ (3.81)	$ (0.18)	$ 0.31	$ (0.94)	$ 0.43	$ 0.07	$ (0.34)	$ 81.88	$ (0.77)	$ (3.14)
Total consolidated operations (in dollars per share)	$ 105.04	$ (2.00)	$ 1.48	$ (3.51)	$ 8.83	$ 6.01	$ 0.28	$ (4.48)	$ 97.44	$ 10.12	$ 53.11
Diluted
Continuing operations (in dollars per share)	$ 16.54	$ 1.81	$ 1.66	$ (3.82)	$ 9.77	$ 5.58	$ 0.21	$ (4.14)	$ 15.56	$ 10.89	$ 56.24
Discontinued operations (in dollars per share)	$ 88.50	$ (3.81)	$ (0.18)	$ 0.31	$ (0.94)	$ 0.43	$ 0.07	$ (0.34)	$ 81.88	$ (0.77)	$ (3.14)
Total consolidated operations (in dollars per share)	$ 105.04	$ (2.00)	$ 1.48	$ (3.51)	$ 8.83	$ 6.01	$ 0.28	$ (4.48)	$ 97.44	$ 10.12	$ 53.10
Adjusted book value per share (in dollars per share)	$ 542.11	$ 436.18	$ 447.69	$ 446.70	$ 440.59	$ 428.79	$ 405.94	$ 410.59