Variable Interest Entity and Equity Method Investments - Investment in SNG (Details) - SNG - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Equity Method Investments [Line Items]
Revenues	$ 146	$ 143	$ 306	$ 298
Operating income	60	63	159	147
Net income	$ 54	$ 48	$ 132	$ 114