Revenue Recognition - Effects of ASC 606 (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 2,149	$ 2,141	$ 6,380	$ 6,436
Cost of services and products (exclusive of depreciation and amortization)	697	742	2,106	2,185
Selling, general and administrative	172	231	602	710
Income tax expense	111	$ 168	322	$ 512
Net income	453		1,260
Other current assets	278		278		$ 98
Other long-term assets, net	126		126
Deferred revenue	377		377
Deferred income taxes, net	1,062		1,062		1,001
Other long-term liabilities	448		448
Accumulated deficit	(231)		(231)		$ (713)
Impact of 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	(13)		(22)
Cost of services and products (exclusive of depreciation and amortization)	3		15
Selling, general and administrative	0		(2)
Income tax expense	(4)		(9)
Net income	(12)		(26)
Other current assets	(268)		(268)
Other long-term assets, net	(20)		(20)
Deferred revenue	(136)		(136)
Deferred income taxes, net	(59)		(59)
Other long-term liabilities	79		79
Accumulated deficit	(172)		(172)
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	2,136		6,358
Cost of services and products (exclusive of depreciation and amortization)	700		2,121
Selling, general and administrative	172		600
Income tax expense	107		313
Net income	441		1,234
Other current assets	10		10
Other long-term assets, net	106		106
Deferred revenue	241		241
Deferred income taxes, net	1,003		1,003
Other long-term liabilities	527		527
Accumulated deficit	$ (403)		$ (403)