Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Tax provision at U.S. federal statutory rate [1]	$ 112,000,000	$ 64,000,000	$ 333,000,000	$ 105,000,000
Tax-exempt interest	(16,000,000)	(31,000,000)	(50,000,000)	(91,000,000)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount	1,000,000	0	8,000,000	0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount	(3,000,000)	(4,000,000)	(5,000,000)	(12,000,000)
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Amount	11,000,000	0	13,000,000	0
Other	(2,000,000)	7,000,000	(2,000,000)	3,000,000
Income tax expense	103,000,000	$ 36,000,000	297,000,000	$ 5,000,000
Unrecognized Tax Benefits, Period Increase (Decrease)	$ 5		$ 5,000,000