Income Taxes (Details) (USD $)	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2011	Sep. 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)		28.00%		26.00%
U.K. blended marginal corporate tax rate (as a percent)					26.50%
Net tax benefit due to filing of amended tax return			$ 55,800,000
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	309,700,000		65,200,000		365,400,000	288,700,000
Tax-exempt interest	(14,400,000)		(19,400,000)		(46,000,000)	(47,800,000)
True-up from tax return filings	(3,100,000)		(51,600,000)		(3,100,000)	(51,600,000)
Change in liability for uncertain tax positions	400,000		(8,000,000)		1,600,000	(6,400,000)
Other	2,300,000		800,000		4,100,000	1,800,000
Total provision (benefit) for income taxes	294,913,000		(12,948,000)		321,959,000	184,724,000
Net deferred tax assets	676,198,000			676,198,000	676,198,000		1,259,125,000
Effective tax rate (as a percent)	27.90%		(8.50%)		27.30%	21.40%
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	1,056,066,000		151,651,000		1,181,188,000	861,918,000
Revenue	1,345,199,000		347,471,000		1,740,406,000	1,436,246,000
United States
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	884,800,000		186,500,000		1,043,700,000	824,600,000
Revenue	1,083,600,000		335,600,000		1,453,000,000	1,232,500,000
Bermuda.
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	171,100,000		(34,600,000)		137,200,000	37,000,000
Revenue	261,500,000		11,900,000		287,300,000	203,800,000
UK
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	100,000		(300,000)		300,000	300,000
Revenue	$ (100,000)