Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 31	$ 116	$ 80	$ 245
Tax-exempt interest	(6)	(12)	(18)	(36)
Bargain purchase gain	0	0	0	(20)
Change in liability for uncertain tax positions	(10)	8	(16)	(27)
Effect of provision to tax return filing adjustment	(1)	(8)	(1)	(8)
State taxes	5	1	6	7
Foreign taxes	1	1	4	4
Taxes on reinsurance	1	(1)	0	(3)
Effect of adjustments to the provisional amount as a result of Tax Act	(4)	0	(4)	0
Other	(3)	0	(5)	(5)
Total provision (benefit) for income taxes	$ 14	$ 105	$ 46	$ 157
Effective tax rate (as a percent)	8.30%	33.60%	9.70%	18.80%
Income (loss) before income taxes	$ 175	$ 313	$ 479	$ 835
Revenue	274	623	788	1,458
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Effect of adjustments to the provisional amount as a result of Tax Act					$ 61
Income (loss) before income taxes	128	337	374	713
Revenue	191	566	628	1,305
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	24	(18)	97	143
Revenue	47	61	135	165
U.K and Other [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	23	(6)	8	(21)
Revenue	$ 36	$ (4)	$ 25	$ (12)