Income Taxes (Details) (USD $)	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
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%
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions										$ 316,000,000	$ 179,200,000	$ 111,900,000
Tax-exempt interest										(61,600,000)	(61,400,000)	(42,600,000)
True-up from tax return filings										(3,200,000)	(51,600,000)
Goodwill												(51,500,000)
Change in liability for uncertain tax positions										2,100,000	(5,600,000)	9,500,000
Change in valuation allowance											(7,000,000)
Other										5,500,000	2,000,000	4,800,000
Total provision (benefit) for income taxes	(63,200,000)	294,800,000	(47,700,000)	74,900,000	(129,100,000)	(13,000,000)	77,900,000	119,800,000		258,842,000	55,599,000	32,110,000
Effective tax rate (as a percent)										25.00%	10.10%	27.50%
Net tax benefit due to filing of amended tax return											55,800,000
Tax benefit related to change in liability for uncertain tax positions due to amended return for a period prior to the AGMH Acquisition											9,200,000
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	(146,800,000)	1,056,000,000	(90,300,000)	215,500,000	(312,600,000)	151,600,000	257,000,000	453,300,000		1,034,462,000	549,306,000	116,932,000
Revenue										1,819,313,000	1,313,453,000	917,159,000
United States
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income										902,700,000	511,600,000	319,600,000
Revenue										1,518,000,000	1,094,500,000	719,800,000
Bermuda.
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income										131,400,000	37,300,000	(202,800,000)
Revenue										301,300,000	219,000,000	197,300,000
UK
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income										$ 300,000	$ 400,000	$ 100,000