Revenue Recognition - Effects of ASC 606 (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 2,101	$ 2,133	$ 4,231	$ 4,295
Cost of services and products (exclusive of depreciation and amortization)	702	723	1,409	1,443
Selling, general and administrative	215	235	430	479
Income tax expense	81	$ 170	211	$ 344
Net income	427		807
Other current assets	222		222		$ 98
Other long-term assets, net	120		120		75
Deferred revenue	341		341
Deferred income taxes, net	1,039		1,039		1,001
Other long-term liabilities	341		341		82
Accumulated deficit	(353)		(353)		(713)
Impact of 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	0		(9)
Cost of services and products (exclusive of depreciation and amortization)	7		12
Selling, general and administrative	(2)		(2)
Income tax expense	(1)		(5)
Net income	(4)		(14)
Other current assets	(114)		(114)
Other long-term assets, net	(67)		(67)
Deferred revenue	14		14
Deferred income taxes, net	(38)		(38)
Other long-term liabilities	(7)		(7)
Accumulated deficit	(150)		(150)		$ 128
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	2,101		4,222
Cost of services and products (exclusive of depreciation and amortization)	709		1,421
Selling, general and administrative	213		428
Income tax expense	80		206
Net income	423		793
Other current assets	108		108
Other long-term assets, net	53		53
Deferred revenue	355		355
Deferred income taxes, net	1,001		1,001
Other long-term liabilities	334		334
Accumulated deficit	$ (503)		$ (503)