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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit)	$ 19	$ 17	$ 50	$ 64
Tax-exempt interest	(5)	(5)	(13)	(15)
Change in liability for uncertain tax positions	(17)	1	(17)	1
Effect of provision to tax return filing adjustment	(7)	(6)	(7)	(6)
State taxes	1	0	2	1
Foreign taxes	(4)	6	3	11
Taxes on reinsurance	4	5	3	9
Deferred compensation	0	(1)	0	(3)
Other	(1)	0	(1)	(1)
Total provision (benefit) for income taxes	$ (10)	$ 17	$ 20	$ 61
Effective tax rate	(12.00%)	19.20%	8.40%	18.60%
Income (loss) before income taxes	$ 72	$ 86	$ 236	$ 323
Total before tax	79	86	239	326
Revenue	268	206	736	667
U.S.
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	72	101	244	326
Revenue	200	186	606	562
Bermuda
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	(14)	10	1	26
Revenue	35	37	94	110
U.K. and other
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before income taxes	21	(25)	(6)	(26)
Revenue	$ 33	$ (17)	$ 36	$ (5)