Investments in Unconsolidated Affiliates (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2011
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	$ 331.9			$ 387.9
Symetra
Investments in unconsolidated affiliates
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates	(17.8)			62.8
Total investments in unconsolidated affiliates	326.0	[1],[2],[3]	377.5	381.5	335.7	[1],[2],[3]	293.2	273.6
Symetra | Common shareholders' equity.
Investments in unconsolidated affiliates
Symetra Common Shares excluding unrealized gains (losses)	343.8			288.4
Total investments in unconsolidated affiliates	326.0	[1],[2],[3]	350.9	351.2	305.9	[1],[2],[3]	269.5	261.0
Symetra | Warrants
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	0	[1],[2],[3]	26.6	30.3	29.8	[1],[2],[3]	23.7	12.6
Hamer, LLC
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	3.5			4.0
Bri-Mar Manufacturing, LLC
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	2.4			1.9
Pentelia Capital Management
Investments in unconsolidated affiliates
Total investments in unconsolidated affiliates	$ 0			$ 0.5

[1] The aggregate value of White Mountains’ investment in common shares of Symetra was $320.6 based upon the quoted market price of $15.99 per share at June 30, 2013.

[2] Includes White Mountains’ equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio of $(17.8) and $29.4 as of June 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $1.4 and $(2.4).

[3] Includes White Mountains’ equity in net unrealized gains from Symetra’s fixed maturity portfolio of $(17.8) and $29.4 as of June 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $1.4 and $(2.4).