Investments in Unconsolidated Affiliates (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012		Jun. 30, 2012	Dec. 31, 2011
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	$ 331.7			$ 387.9
Symetra
Investments in unconsolidated affiliates
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates	(25.5)			62.8
Total investments in unconsolidated affiliates	325.2	[1],[2],[3]	326.0	381.5	374.8	[1],[2],[3]	335.7	273.6
Symetra | Common shareholders' equity.
Investments in unconsolidated affiliates
Symetra Common Shares excluding unrealized gains (losses)	350.7			288.4
Total investments in unconsolidated affiliates	325.2	[1],[2],[3]	326.0	351.2	348.6	[1],[2],[3]	305.9	261.0
Symetra | Warrants
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	0	[1],[2],[3]	0	30.3	26.2	[1],[2],[3]	29.8	12.6
Hamer, LLC
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	3.7			4.0
Bri-Mar Manufacturing, LLC
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	2.8			1.9
Pentelia Capital Management
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	$ 0			$ 0.5

[1] Includes White Mountains’ equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio of $(25.5) and $64.6 as of September 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $2.0 and $(5.2).

[2] The aggregate value of White Mountains’ investment in common shares of Symetra was $357.3 based upon the quoted market price of $17.82 per share at September 30, 2013.

[3] Includes White Mountains’ equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio of $(7.7) and $35.2 as of September 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $0.5 and $2.8.