Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended							10 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Oct. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Operating Revenue		$ 1,407	$ 2,071	$ 2,010	$ 2,052	$ 2,087				$ 6,870	$ 8,220	$ 8,277	$ 8,173
Operating Income		158	284	227	196	261					968	1,309
Net income (loss)		(141)	$ 151	$ 88	$ 40	$ 62					341	284
Tax Cuts and Jobs Act, provisional tax expense		$ 195									$ 92	$ 195
Predecessor
Selected Quarterly Financial Information [Abstract]
Operating Revenue	$ 701						$ 2,059	$ 2,062	$ 2,048	6,870			8,173
Operating Income	112						349	353	337	1,151			1,445
Net income (loss)	$ 19						$ 157	$ 154	$ 95	$ 425			$ 677