Joint Ownership Agreements - Investment in SNG (Details) - SNG - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Revenues	$ 82	$ 146	$ 445
Operating income	60	71	218
Net income	$ 55	$ 57	$ 172