Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current tax expense (benefit):
Federal	$ 56	$ (122)	$ 122
State	(2)	0	0
Total current tax expense (benefit)	54	(122)	122
Deferred tax expense (benefit):
Federal	1	859	(152)
State	0	3	1
Total deferred tax expense (benefit)	1	862	(151)
Total income tax expense (benefit)	$ 55	$ 740	$ (29)