Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Tax provision at U.S. federal statutory rate [1]	$ 112	$ (99)	$ 221	$ 41
Tax-exempt interest	(17)	(30)	(34)	(60)
Effective Income Tax Rate Reconciliation, Share-based Compensation Arrangements Vesting, Amount	3	0	7	0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount	0	(1)	(2)	(8)
Other	5	1	2	(4)
Income tax expense	$ 103	$ (129)	$ 194	$ (31)