Income Taxes (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Income Taxes
U.S. marginal corporate income tax rate (as a percent)				35.00%
U.K. marginal corporate tax rate (as a percent)	26.00%	28.00%		26.50%
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ (33,000,000)		$ 91,000,000	$ 55,700,000	$ 223,500,000
Tax-exempt interest	(16,200,000)		(14,200,000)	(31,600,000)	(28,300,000)
Change in liability for uncertain tax positions	600,000		600,000	1,100,000	1,100,000
Other	900,000		500,000	2,000,000	1,400,000
Total provision (benefit) for income taxes	(47,820,000)		77,826,000	27,046,000	197,672,000
Net deferred tax assets	1,031,438,000			1,031,438,000		1,259,125,000
Effective tax rate (as a percent)	52.90%		30.30%	21.60%	27.80%
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	(90,356,000)		256,961,000	125,122,000	710,267,000
Revenue	116,294,000		424,756,000	395,207,000	1,088,775,000
United States
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	(94,400,000)		246,400,000	158,900,000	625,300,000
Revenue	75,700,000		368,100,000	369,400,000	896,900,000
Bermuda.
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	4,000,000		10,100,000	(33,900,000)	84,400,000
Revenue	40,600,000		56,600,000	25,800,000	191,900,000
UK
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	100,000		500,000	200,000	600,000
Revenue				$ 100,000