Discontinued Operations (Details) (USD $) In Millions							3 Months Ended		6 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Jun. 30, 2011 White Mountains Re Segment	Jun. 30, 2010 White Mountains Re Segment	Jun. 30, 2011 White Mountains Re Segment	Jun. 30, 2010 White Mountains Re Segment	Jun. 30, 2011 Discontinued Operations.	Dec. 31, 2010 Discontinued Operations.
Discontinued Operations
Investment income and realized and unrealized investment gains and losses previously included in the Esurance segment excluded from net income (loss) from discontinued operations							$ 0.9	$ 1.7	$ (3.3)	$ 2.8
Assets held for sale
Fixed maturity investments, at fair value	5,558.9		5,786.1								761.2	593.8
Short-term investments, at amortized cost (which approximates fair value)	813.5		975.3								24.6	131.0
Common equity securities, at fair value	703.8		667.0								43.9	43.0
Convertible fixed maturity investments, at fair value	119.7		143.0								15.0	17.2
Other long-term investments	333.3		372.1									4.0
Total investments	7,529.2		7,943.5								844.7	789.0
Cash	483.7		395.0								57.3	39.8
Reinsurance recoverable on paid and unpaid losses											34.8	34.4
Insurance premiums receivable	672.0		532.5								107.8	102.9
Deferred acquisition costs	194.1		176.1								43.0	34.7
Deferred tax asset	407.4		431.9								74.4	67.8
Other assets	679.3		730.0								51.9	54.2
Total assets	14,202.5		14,534.1								1,213.9	1,122.8
Liabilities held for sale
Loss and loss adjustment expense reserves	5,642.4	5,759.8	5,736.8	6,197.9	6,660.7	6,379.2					456.1	450.4
Unearned insurance premiums	1,033.8		938.7								215.3	201.6
Accrued incentive compensation	153.3		157.4								22.5	19.1
Other liabilities	1,089.1		1,167.7								86.0	80.4
Total liabilities	9,986.7		10,273.3								779.9	751.5
Net assets held for sale											$ 434.0	$ 371.3