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
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit)	$ 17	$ 31	$ 64	$ 80
Tax-exempt interest	(5)	(6)	(15)	(18)
Change in liability for uncertain tax positions	1	(10)	1	(16)
Effect of provision to tax return filing adjustment	(6)	(1)	(6)	(1)
State tax	0	5	1	6
Foreign taxes	6	1	11	4
Taxes on reinsurance	5	1	9	0
Effect of adjustments to the provisional amounts as a result of 2017 Tax Cuts and Jobs Act	0	(4)	0	(4)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Deferred Compensation, Amount	(1)	(1)	(3)	(2)
Other	0	(2)	(1)	(3)
Provision (benefit) for income taxes	$ 17	$ 14	$ 61	$ 46
Effective tax rate (as a percent)	19.20%	8.30%	18.60%	9.70%
Income (loss) before income taxes	$ 86	$ 176	$ 323	$ 479
Total before tax	86	175	326	479
Revenue	206	275	667	788
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	101	128	326	374
Revenue	186	192	562	628
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	10	24	26	97
Revenue	37	47	110	135
U.K and Other [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	(25)	23	(26)	8
Revenue	$ (17)	$ 36	$ (5)	$ 25