Revenue Recognition - Comparative Results (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Operating revenue	$ 2,012	$ 2,039	$ 2,051	$ 2,055	$ 2,113	$ 2,149	$ 2,101	$ 2,130	$ 8,157	$ 8,493	$ 8,550
Cost of services and products (exclusive of depreciation and amortization)									2,438	2,767	2,881
Selling, general and administrative									659	799	925
Income tax expense									$ 641	494	$ 134
Net income										1,665
ASC 605 Historical Adjusted Amount
Income Statement [Abstract]
Operating revenue										8,499
Cost of services and products (exclusive of depreciation and amortization)										2,784
Selling, general and administrative										799
Income tax expense										491
Net income										1,657
Accounting Standards Update 2014-09 | Impact of ASC 606
Income Statement [Abstract]
Operating revenue										6
Cost of services and products (exclusive of depreciation and amortization)										17
Selling, general and administrative										0
Income tax expense										(3)
Net income										$ (8)