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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Jul. 31, 2018	Dec. 31, 2019	Dec. 31, 2017
Current Income Taxes
Federal										$ 0	$ (14)	$ 19	$ 52
State										0	(1)	74	7
Total current income taxes										0	(15)	93	59
Deferred Income Taxes
Federal										(1,015)	54	(298)	(43)
State										(6)	9	(68)	(3)
Total deferred income taxes										(1,021)	63	(366)	(46)
Total income tax (benefit) expense	$ (19)	$ (979)	$ (221)	$ 24	$ (29)	$ (47)	$ (42)	$ 21	$ 46	$ (1,021)	$ 48	$ (273)	$ 13