Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Balance as of January 1	$ 2,293	$ 2,286
Goodwill attributable to Gas Storage and Pipelines 2016 acquisition of AGS and SGG	0	7
Balance at December 31	$ 2,293	$ 2,293