Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ 35	$ 85
Tax-exempt interest	(6)	(12)
Goodwill impairment and gain on bargain purchase price	0	(20)
Change in liability for uncertain tax positions	(7)	0
Foreign taxes	(4)	0
State taxes	2	5
Other	0	(3)
Total provision (benefit) for income taxes	$ 20	$ 55
Effective tax rate (as a percent)	9.30%	14.70%
Income (loss) before income taxes	$ 217	$ 372
Revenue	293	527
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	175	246
Revenue	247	474
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	49	133
Revenue	52	57
United Kingdom [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	(7)	(7)
Revenue	$ (6)	$ (4)