Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Mar. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Pretax Income and Revenue by Tax Jurisdiction
Net tax benefit due to amended tax return														$ 56
Net tax benefit due to amended tax return portion related to change in liability for uncertain tax positions														9
Income (loss) before provision for income taxes	95	179	554	(696)	(148)	1,055	(91)	213	132			1,029		534
Revenue									973			1,819		1,313
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions									76			313		173
Tax-exempt interest									(61)			(62)		(61)
True-up from tax return filings(1)									0	[1]		(3)	[1]	(52)	[1]
Change in liability for uncertain tax positions									2			2		(5)
Change in valuation allowance									0			0		(7)
Other									5			6		2
Total provision (benefit) for income taxes	21	37	177	(213)	(64)	294	(48)	74	22			256		50
Effective tax rate (as a percent)									16.50%			24.90%		9.40%
Effective income tax rate reconciliation US income tax rate (as a percent)									35.00%
U.K. marginal corporate tax rate (as a percent)	24.00%							28.00%	24.50%		26.00%	26.50%
United States
Pretax Income and Revenue by Tax Jurisdiction
Income (loss) before provision for income taxes									218			896		496
Revenue									894			1,518		1,094
Bermuda
Pretax Income and Revenue by Tax Jurisdiction
Income (loss) before provision for income taxes									(86)			133		38
Revenue									79			301		219
UK
Pretax Income and Revenue by Tax Jurisdiction
Income (loss) before provision for income taxes									0			0		0
Revenue									$ 0			$ 0		$ 0

[1] For the year ended December 31, 2010, the Company recorded a $56 million tax benefit related to an amended return for a period prior to the AGMH Acquisition, $9 million was related to a change in liability for uncertain tax positions.