Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax expense (benefit):
State		$ 13	$ (11)	$ 3
Foreign		0	0	2
Total current		13	(11)	5
Deferred income tax expense (benefit):
Federal		266	399	285
State		73	51	(10)
Total deferred		339	450	275
Non-current tax expense (benefit)		14	11	6
Income tax expense (benefit)		$ 366	450	286
Federal statutory tax rate related to federal tax reform law		21.00%
Additional tax expense related to the federal tax reform law	$ 58		58
Unrecognized Tax Benefits	$ 25	$ 31	$ 25	$ 28	$ 29