Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	2 Months Ended	3 Months Ended								10 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Oct. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
Operating Revenue	$ 1,407	$ 2,061	$ 2,064	$ 2,014	$ 2,046	$ 2,071	$ 2,010	$ 2,052	$ 2,087	$ 6,870	$ 8,185	$ 8,220
Operating Income	158	285	309	272	(3,393)	284	227	196	261	1,151	(2,527)	968
Net (loss) income	(141)	$ 160	$ 114	$ 110	(3,585)	$ 151	$ 88	$ 40	$ 62	425	(3,201)	341
Goodwill impairment	$ 0				$ 3,708					$ 0	$ 3,708	$ 0