Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax expense (benefit):
Federal	$ (122)	$ 122	$ 202
State	0	0	(11)
Total current tax expense (benefit)	(122)	122	191
Deferred tax expense (benefit):
Federal	859	(152)	(104)
State	3	1	(3)
Total deferred tax expense (benefit)	862	(151)	(107)
Total income tax expense (benefit)	$ 740	$ (29)	$ 84