Revenue Recognition - Effects of ASC 606 (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 2,052	$ 4,139
Cost of services and products (exclusive of depreciation and amortization)	980	1,978
Selling, general and administrative	388	732
Income tax expense	44	146
Net income	40	102
Other current assets	201	201	$ 117
Deferred income taxes	281	281
Other long-term assets, net	108	108	63
Member's equity	18,749	18,749	$ 19,254
Impact of 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	0	0
Cost of services and products (exclusive of depreciation and amortization)	0	0
Selling, general and administrative	7	20
Income tax expense	(2)	(5)
Net income	(5)	(15)
Other current assets	(16)	(16)
Deferred income taxes	9	9
Other long-term assets, net	(18)	(18)
Member's equity	(25)	(25)
ASC 605 Historical Adjusted Balances
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	2,052	4,139
Cost of services and products (exclusive of depreciation and amortization)	980	1,978
Selling, general and administrative	395	752
Income tax expense	42	141
Net income	35	87
Other current assets	185	185
Deferred income taxes	290	290
Other long-term assets, net	90	90
Member's equity	$ 18,724	$ 18,724