Revenue Recognition - Reported Results Under ASC 606 (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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
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
Cost of services and products (exclusive of depreciation and amortization)	690									3,493	3,799	3,937
Selling, general and administrative	253									1,208	1,258	1,354
Interest expense	80									441	502	509
Income tax expense	234									268	255	196
Net (loss) income	$ (141)	$ 160	$ 114	$ 110	$ (3,585)	$ 151	$ 88	$ 40	$ 62	$ 425	$ (3,201)	341
Impact of ASC 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating Revenue												(5)
Cost of services and products (exclusive of depreciation and amortization)												0
Selling, general and administrative												52
Interest expense												(9)
Income tax expense												(12)
Net (loss) income												(36)
ASC 605 Historical Adjusted Balances
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating Revenue												8,215
Cost of services and products (exclusive of depreciation and amortization)												3,937
Selling, general and administrative												1,406
Interest expense												500
Income tax expense												184
Net (loss) income												$ 305