Investments in Unconsolidated Affiliates (Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended											1 Months Ended	12 Months Ended			1 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		Apr. 30, 2007 Pentelia Capital Management	Dec. 31, 2012 Pentelia Capital Management	Dec. 31, 2011 Pentelia Capital Management	Dec. 31, 2010 Pentelia Capital Management	Dec. 31, 2010 Delos	Aug. 31, 2006 Delos	Dec. 31, 2012 Hamer and Bri-Mar
Symetra balance sheet data; [Abstract]
Total investments												$ 27,556.4		$ 26,171.7
Separate account assets												807.7		795.8
Total assets												29,460.9	[1]	28,183.3	[1]
Policyholder liabilities												23,735.2		23,140.6
Long-term debt												449.4		449.2
Separate account liabilities												807.7		795.8
Total liabilities												25,830.8	[1]	25,068.4	[1]
Common shareholders’ equity												3,630.1	[1]	3,114.9	[1]
Symetra income statement data: [Abstract]
Net premiums earned												605.0		540.5		473.0
Net investment income												1,275.2		1,270.9		1,199.4
Total revenues												2,101.2	[2]	1,999.3	[2]	1,878.8	[2]
Policy benefits												1,371.8		1,307.3		1,234.6
Total expenses												1,831.1	[2]	1,726.1	[2]	1,600.5	[2]
Net income												205.4	[2]	195.8	[2]	193.8	[2]
Comprehensive net income												549.3		785.5		679.0
Ownership interest (as a percent)												15.00%								33.00%					18.00%
Payments to acquire investment																				1.6					32.0
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	[3],[4],[5]	28.2	[3],[4]	11.1	[3],[4],[6]				(1.3)	(0.2)	0.5			0.4
Investments in unconsolidated affiliates	387.9				275.3				387.9	275.3		381.5	[7],[8]	273.6	[7]	387.5	[7]	307.7	[7]		0.5	1.7
Net sales proceeds																								21.7	138.8
Gain (loss) on dsposal																								$ (10.9)	$ 14.0

[1] 2011 balances have been restated for the effect of Symetra’s adoption of ASU 2010-26

[2] Amounts for the years ended December 31, 2011 and 2010 have been restated for the effect of Symetra’s adoption of ASU 2010-26.

[3] 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

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

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

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

[7] 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

[8] 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.