Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Balance as of January 1	$ 2,018	$ 2,018
Goodwill attributable to Gas Storage and Pipelines acquisition	268	0
Balance at December 31	$ 2,286	$ 2,018