Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Balance as of January 1	$ 2,286	$ 2,018
Goodwill attributable to Gas Storage and Pipelines acquisition	7	268
Balance at December 31	$ 2,293	$ 2,286