Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended									9 Months Ended	12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating Loss Carryforwards [Line Items]
U.K. marginal corporate tax rate (as a percent)								26.00%	28.00%	23.00%	23.25%	24.00%	24.50%	26.50%
Expected tax provision (benefit) at statutory rates in taxable jurisdictions											$ 390		$ 76	$ 313
Tax-exempt interest											(57)		(61)	(62)
Change in liability for uncertain tax positions											(2)		2	2
Other											3		5	3
Total provision (benefit) for income taxes	140	152	110	(68)	21	37	177	(213)			334		22	256
Effective tax rate (as a percent)											29.20%		16.50%	24.90%
Income (loss) before provision for income taxes	489	536	329	(212)	95	179	554	(696)			1,142		132	1,029
Revenue											1,608		954	1,805
United States
Operating Loss Carryforwards [Line Items]
Income (loss) before provision for income taxes											1,118		218	896
Revenue											1,389		875	1,504
Bermuda
Operating Loss Carryforwards [Line Items]
Income (loss) before provision for income taxes											27		(86)	133
Revenue											219		79	301
United Kingdom
Operating Loss Carryforwards [Line Items]
Income (loss) before provision for income taxes											(3)		0	0
Revenue											$ 0		$ 0	$ 0