Revenue Recognition - Effects of ASC 606 (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 2,010	$ 6,149
Cost of services and products (exclusive of depreciation and amortization)	976	2,954
Selling, general and administrative	311	1,043
Interest expense	137	381
Income tax expense	38	184
Net income	88	190
Other current assets	290	290	$ 117
Other long-term assets, net	132	132	63
Deferred revenue	1,469	1,469
Deferred income tax assets, net	274	274
Member's equity	18,312	18,312	$ 19,254
Impact of 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	(4)	(4)
Cost of services and products (exclusive of depreciation and amortization)	0	0
Selling, general and administrative	12	32
Interest expense	(7)	(7)
Income tax expense	(2)	(8)
Net income	(7)	(21)
Other current assets	(22)	(22)
Other long-term assets, net	(22)	(22)
Deferred revenue	(3)	(3)
Deferred income tax assets, net	10	10
Member's equity	(31)	(31)
ASC 605 Historical Adjusted Balances
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	2,006	6,145
Cost of services and products (exclusive of depreciation and amortization)	976	2,954
Selling, general and administrative	323	1,075
Interest expense	130	374
Income tax expense	36	176
Net income	81	169
Other current assets	268	268
Other long-term assets, net	110	110
Deferred revenue	1,466	1,466
Deferred income tax assets, net	284	284
Member's equity	$ 18,281	$ 18,281