Income Taxes - Schedule of income tax benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 42	$ 0	$ 0
State	1	0	(1)
Change in income tax contingencies	(1)	(2)	0
Total current income tax expense (benefit)	42	(2)	(1)
Deferred:
Federal	(102)	(100)	(79)
State	(19)	(9)	(2)
Total deferred income tax benefit	(121)	(109)	(81)
Income tax benefit	$ (79)	$ (111)	$ (82)