Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Pretax Income and Revenue by Tax Jurisdiction
Interest on income taxes expense	$ 3
Unrecognized Tax Benefits	14		22	20
Income (loss) before provision for income taxes	(212)	(696)
Revenue	(176)	(362)
Increase in unrecognized tax benefits as a result of position taken during the current period	1		2
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	(48)	(199)
Tax-exempt interest	(14)	(15)
Change in liability for uncertain tax positions	(8)	0
Other	2	1
Total provision (benefit) for income taxes	(68)	(213)
Effective tax rate (as a percent)	31.80%	30.60%
United States
Pretax Income and Revenue by Tax Jurisdiction
Income (loss) before provision for income taxes	(134)	(568)
Revenue	(127)	(285)
Bermuda
Pretax Income and Revenue by Tax Jurisdiction
Income (loss) before provision for income taxes	(78)	(128)
Revenue	(49)	(77)
UK
Pretax Income and Revenue by Tax Jurisdiction
Income (loss) before provision for income taxes	0	0
Revenue	$ 0	$ 0
Effective Tax Rate Reconciliation
Federal statutory income tax rate (as a percent)	35.00%
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%