Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Tax provision at U.S. federal statutory rate	$ 109	$ 140
Tax-exempt interest	(17)	(30)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount	4	0
Effective Income Tax Rate Reconciliation, Share-based Compensation Arrangements Vesting, Amount	(2)	(7)
Tax Cuts and Jobs Act of 2017, Deferred Tax Asset, Income Tax Expense (Benefit)	(3)	0
Other	0	(5)
Income tax expense	$ 91	$ 98