Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 124	$ 191	$ 76	$ (8)
Tax-exempt interest	(15)	(16)	(29)	(31)
Change in liability for uncertain tax positions	1	1	(7)	1
Other	0	1	2	2
Total provision (benefit) for income taxes	110	177	42	(36)
Effective tax rate (as a percent)	33.50%	32.00%	36.40%	25.00%
Income (loss) before provision for income taxes	329	554	117	(142)
Revenue	465	755	289	393
United States
Effective Tax Rate Reconciliation
Income (loss) before provision for income taxes	353	546	219	(22)
Revenue	425	676	298	391
Bermuda
Effective Tax Rate Reconciliation
Income (loss) before provision for income taxes	(24)	8	(102)	(120)
Revenue	40	79	(9)	2
UK
Effective Tax Rate Reconciliation
Federal statutory income tax rate (as a percent)	35.00%
Income (loss) before provision for income taxes	0	0	0	0
Revenue	$ 0	$ 0	$ 0	$ 0
UK | Subsequent to April 1, 2013
Effective Tax Rate Reconciliation
Federal statutory income tax rate (as a percent)	23.00%
UK | April 1, 2012 to April 1, 2013
Effective Tax Rate Reconciliation
Effective tax rate (as a percent)	23.25%
Federal statutory income tax rate (as a percent)	24.00%
UK | Prior to April 1, 2012
Effective Tax Rate Reconciliation
Effective tax rate (as a percent)	24.50%
Federal statutory income tax rate (as a percent)	26.00%