Investments in Unconsolidated Affiliates (Details) (USD $) In Millions, unless otherwise specified			3 Months Ended				9 Months Ended				12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Symetra		Sep. 30, 2011 Symetra		Sep. 30, 2012 Symetra		Sep. 30, 2011 Symetra		Dec. 31, 2011 Symetra	Jun. 30, 2012 Symetra	Mar. 31, 2012 Symetra	Dec. 31, 2010 Symetra	Sep. 30, 2012 Symetra Common shareholders' equity.		Jun. 30, 2012 Symetra Common shareholders' equity.	Dec. 31, 2011 Symetra Common shareholders' equity.	Sep. 30, 2011 Symetra Common shareholders' equity.		Jun. 30, 2011 Symetra Common shareholders' equity.	Dec. 31, 2010 Symetra Common shareholders' equity.	Sep. 30, 2012 Symetra Warrants		Mar. 31, 2012 Symetra Warrants	Dec. 31, 2011 Symetra Warrants	Sep. 30, 2011 Symetra Warrants		Mar. 31, 2011 Symetra Warrants	Dec. 31, 2010 Symetra Warrants	Sep. 30, 2012 Pentelia Capital Management	Dec. 31, 2011 Pentelia Capital Management
Investments in unconsolidated affiliates
Symetra Common Shares excluding unrealized gains (losses)															$ 284.0			$ 261.0
Total investments in unconsolidated affiliates	376.3	275.3	374.8	[1],[2]	456.7	[1],[2]	374.8	[1],[2]	456.7	[1],[2]	273.6	421.0	335.7	387.5	348.6	[1],[2]	305.9	261.0	449.0	[1],[2]	388.5	350.4	26.2	[1],[2]	29.8	12.6	7.7	[1],[2]	32.5	37.1	1.5	1.7
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates			$ 35.2		$ 147.7		$ 64.6		$ 147.7		$ 0

[1] 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.

[2] 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.