Income Taxes - Income Tax Expense (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended							5 Months Ended	7 Months Ended	12 Months Ended
	Jul. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Jul. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current Income Taxes
Federal										$ 0
State										0
Total current income taxes										0
Deferred Income Taxes
Federal										(1,015)
State										(6)
Total deferred income taxes										(1,021)
Total income tax (benefit) expense		$ (979)	$ (42)							$ (1,021)
Predecessor
Current Income Taxes
Federal											$ (14)	$ 52	$ 14
State											(1)	7	4
Total current income taxes											(15)	59	18
Deferred Income Taxes
Federal											54	(43)	(4)
State											9	(3)	(1)
Total deferred income taxes											63	(46)	(5)
Total income tax (benefit) expense	$ (19)			$ 21	$ 46	$ 17	$ 5	$ (10)	$ 1		$ 48	$ 13	$ 13