Income Taxes - Schedule of Components of Income Tax Provision (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current expense:
Federal	$ 539	$ 959	$ 585
State	80	97	99
Total current expense	619	1,056	684
Deferred expense:
Federal	253	(14)	99
State	39	16	11
Total deferred expense	292	2	110
Provision for income taxes	$ 911	$ 1,058	$ 794