Revenue Recognition - Comparative Results (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Operating revenue	$ 2,113	$ 2,149	$ 2,101	$ 2,130	$ 2,114	$ 2,141	$ 2,133	$ 2,162	$ 8,493	$ 8,550	$ 8,910
Cost of services and products (exclusive of depreciation and amortization)									2,767	2,881	2,934
Selling, general and administrative									799	925	1,020
Income tax expense	172	$ 111	$ 81	$ 130	(378)	$ 168	$ 170	$ 174	494	134	$ 678
Net income									1,665
Balance Sheet
Other current assets	147								147
Other long-term assets, net	96								96
Deferred revenue	303								303
Deferred income taxes, net	1,098				1,001				1,098	1,001
Other long-term liabilities	547								547
Accumulated deficit	(182)				$ (713)				(182)	$ (713)
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
Balance Sheet
Other current assets	28								28
Other long-term assets, net	42								42
Deferred revenue	290								290
Deferred income taxes, net	1,045								1,045
Other long-term liabilities	589								589
Accumulated deficit	(331)								(331)
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)
Balance Sheet
Other current assets	(119)								(119)
Other long-term assets, net	(54)								(54)
Deferred revenue	(13)								(13)
Deferred income taxes, net	(53)								(53)
Other long-term liabilities	42								42
Accumulated deficit	$ (149)								$ (149)