Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 143	$ 72	$ 220	$ 110
Tax-exempt interest	(13)	(14)	(27)	(28)
Gain on bargain purchase	(19)	0	(19)	0
Change in liability for uncertain tax positions	1	0	2	1
Other	0	1	1	3
Total provision (benefit) for income taxes	$ 112	$ 59	$ 177	$ 86
Effective tax rate (as a percent)	27.50%	27.20%	26.20%	30.00%
Income (loss) before provision for income taxes	$ 409	$ 218	$ 675	$ 287
Revenue	695	353	1,064	548
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes	414	209	637	322
Revenue	618	293	918	488
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes	5	18	55	(19)
Revenue	78	61	151	62
United Kingdom [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes	(10)	(9)	(17)	(16)
Revenue	$ (1)	$ (1)	$ (5)	$ (2)