Income Taxes (Details) (USD $)	3 Months Ended					6 Months Ended				12 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Mar. 31, 2011	Jun. 30, 2012		Jun. 30, 2011		Mar. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation US Income Tax Rate						35.00%
U.K. marginal corporate tax rate (as a percent)	24.00%				28.00%	24.50%				26.00%	26.50%
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 191,200,000		$ (33,600,000)			$ (7,600,000)		$ 54,100,000
Tax-exempt interest	(15,500,000)		(16,200,000)			(30,900,000)		(31,500,000)
Change in liability for uncertain tax positions	500,000		600,000			1,100,000		1,200,000
Other	900,000		1,000,000			1,900,000		1,900,000
Total provision (benefit) for income taxes	177,105,000		(48,269,000)			(35,490,000)		25,663,000
Effective tax rate (as a percent)	32.00%		52.80%			25.00%		21.10%
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income (Loss)	553,653,000		(91,374,000)			(141,966,000)		121,814,000
Revenue	759,498,000		116,294,000			402,092,000		395,207,000
Period for which cumulative operating income was considered for assessment of realization of deferred tax assets (in years)	3 years					3 years
United States
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income (Loss)	546,300,000		(95,700,000)			(21,900,000)		155,000,000
Revenue	680,100,000	[1]	75,700,000	[1]		399,700,000	[1]	369,400,000	[1]
Bermuda
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income (Loss)	7,200,000		4,300,000			(120,400,000)		(33,300,000)
Revenue	79,400,000	[1]	40,600,000	[1]		2,400,000	[1]	25,800,000	[1]
United Kingdom
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income (Loss)	100,000		100,000			300,000		200,000
Revenue	$ 0	[1]	$ 0	[1]		$ 0	[1]	$ 100,000	[1]

[1] In the above tables, pretax income and revenues of the Company’s subsidiaries which are not U.S. domiciled but are subject to U.S. tax by election or as controlled foreign corporations are included in the U.S. amounts.