Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended				9 Months Ended				12 Months Ended		3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Symetra		Sep. 30, 2011 Symetra		Sep. 30, 2012 Symetra		Sep. 30, 2011 Symetra		Dec. 31, 2011 Symetra	Mar. 31, 2012 Symetra	Sep. 30, 2012 Symetra Common shareholders' equity.		Sep. 30, 2011 Symetra Common shareholders' equity.		Sep. 30, 2012 Symetra Common shareholders' equity.		Sep. 30, 2011 Symetra Common shareholders' equity.		Sep. 30, 2012 Symetra Warrants		Sep. 30, 2011 Symetra Warrants		Sep. 30, 2012 Symetra Warrants		Sep. 30, 2011 Symetra Warrants		Mar. 31, 2012 Symetra Warrants	Mar. 31, 2011 Symetra Warrants
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period			$ 275.3		$ 421.0				$ 273.6		$ 387.5		$ 387.5	$ 335.7	$ 305.9		$ 388.5		$ 261.0		$ 350.4						$ 12.6		$ 37.1		$ 29.8	$ 32.5
Equity in earnings	7.7	1.5	24.4	16.1	8.8	[1],[2]	1.6	[1],[2]	26.7	[1],[2]	17.5	[1],[2]			8.8	[1],[2]	1.6	[1],[2]	26.7	[1],[2]	17.5	[1],[2]	0	[1],[2]	0	[1],[2]	0	[1],[2]	0	[1],[2]
Equity in net unrealized losses from Symetra's fixed maturity portfolio					35.2	[3]	59.9	[3]	64.6	[3]	84.0	[3]			35.2	[3]	59.9	[3]	64.6	[3]	84.0	[3]	0	[3]	0	[3]	0	[3]	0	[3]
Dividends received					(1.3)		(1.0)		(3.7)		(2.9)				(1.3)		(1.0)		(3.7)		(2.9)		0		0		0		0
Increase (decrease) in value of warrants					(3.6)		(24.8)		13.6		(29.4)				0		0		0		0		(3.6)		(24.8)		13.6		(29.4)
Carrying value at the end of the period	376.3		376.3		374.8	[4],[5]	456.7	[4],[5]	374.8	[4],[5]	456.7	[4],[5]	273.6	335.7	348.6	[4],[5]	449.0	[4],[5]	348.6	[4],[5]	449.0	[4],[5]	26.2	[4],[5]	7.7	[4],[5]	26.2	[4],[5]	7.7	[4],[5]	29.8	32.5
Tax on equity in earnings					0.7		0.1		2.2		1.4
Equity in earnings increase relating to amortization of common share impairment															0.9				0.9
Equity in net unrealized losses increase relating to amortization of common share impairment													136.6		3.3				9.9
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates					35.2		147.7		64.6		147.7		0
Tax on equity in net unrealized gains (losses)					2.8		12.3		5.2		24.2
Aggregate value of White Mountains common shares of Symetra at quoted market price					214.0				214.0
Quoted market price (in dollars per share)					$ 12.30				$ 12.30
Cash dividends received on warrants																											$ 0.7		$ 2.0

[1] Equity in earnings excludes tax expense of $2.2 and $1.4.

[2] Equity in earnings includes $0.9 increase relating to the pre-tax amortization of Symetra common share impairment from September 30, 2012.

[3] Net unrealized gains includes $9.9 increase relating to the pre-tax amortization of Symetra common share impairment from September 30, 2012

[4] Includes White Mountains’ equity in net unrealized gains from Symetra’s fixed maturity portfolio of $64.6 and $147.7 as of September 30, 2012 and 2011, which exclude tax expense of $5.2 and $24.2.

[5] The aggregate value of White Mountains’ investment in common shares of Symetra was $214.0 based upon the quoted market price of $12.30 per share at September 30, 2012.