Investments in Unconsolidated Affiliates (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended											12 Months Ended								12 Months Ended																12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Symetra		Dec. 31, 2011 Symetra		Dec. 31, 2010 Symetra		Dec. 31, 2009 Symetra		Dec. 31, 2012 Symetra Common shareholders’ equity		Dec. 31, 2011 Symetra Common shareholders’ equity		Dec. 31, 2010 Symetra Common shareholders’ equity		Dec. 31, 2009 Symetra Common shareholders’ equity		Dec. 31, 2012 Symetra Warrants		Dec. 31, 2011 Symetra Warrants		Dec. 31, 2010 Symetra Warrants		Dec. 31, 2009 Symetra Warrants		Dec. 31, 2012 Hamer, LLC		Dec. 31, 2011 Hamer, LLC		Dec. 31, 2012 Bri-Mar Manufacturing, LLC		Dec. 31, 2011 Bri-Mar Manufacturing, LLC		Dec. 31, 2012 Pentelia Capital Management	Dec. 31, 2011 Pentelia Capital Management	Dec. 31, 2010 Pentelia Capital Management
Investments in unconsolidated affiliates
Equity in earnings (losses) of unconsolidated affiliates	$ 5.5	$ 7.7	$ 6.5	$ 10.2	$ (36.4)	$ 1.5	$ 7.9	$ 6.8	$ 29.9	$ (20.2)	$ 9.9	$ 32.3	[1],[2],[3]	$ 28.2	[1],[2]	$ 11.1	[1],[2],[4]			$ 32.3	[1],[2],[3]	$ 28.2	[1],[2]	$ 11.1	[1],[2],[4]			$ 0	[1],[2],[3]	$ 0	[1],[2]	$ 0	[1],[2],[4]											$ (1.3)	$ (0.2)	$ 0.5
Symetra common shares																				288.4		261.0
Unrealized gains from Symetra’s fixed maturity portfolio												62.8		0		63.7
Total investment in Symetra	$ 387.9				$ 275.3				$ 387.9	$ 275.3		$ 381.5	[5],[6]	$ 273.6	[5]	$ 387.5	[5]	$ 307.7	[5]	$ 351.2	[5],[6]	$ 261.0	[5]	$ 350.4	[5]	$ 269.2	[5]	$ 30.3	[5],[6]	$ 12.6	[5]	$ 37.1	[5]	$ 38.5	[5]	$ 4.0	[7]	$ 0	[7]	$ 1.9	[7]	$ 0	[7]	$ 0.5	$ 1.7

[1] Equity in earnings for the years end December 31, 2012, 2011 and 2010 includes $1.3, $1.0, and $0.8 loss from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra

[2] Equity in earnings for the years end December 31, 2012, 2011 and 2010 excludes tax expense of $2.6, $2.3, and $1.4

[3] Equity in earnings includes $3.5 increase relating to the pre-tax amortization of Symetra common share impairment from December 31, 2012.

[4] Includes a $17.9 loss from the dilutive effect of Symetra’s public offering

[5] Includes White Mountains’ equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio of $62.8, $0, and $63.7 as of December 31, 2012, 2011 and 2010, which excludes tax expense of $5.1, $0 and $5.2

[6] The aggregate value of White Mountains’ investment in common shares of Symetra was $225.9 based upon the quoted market price of $12.98 per share at December 31, 2012.

[7] As of October 1, 2012, Hamer and Bri-Mar are no longer consolidated and are accounted for as investments in unconsolidated affiliates.