Goodwill (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($)
Goodwill [Line Items]
Goodwill, Beginning Balance	$ 862
Goodwill, Acquired During Period	5	[1]
Goodwill, Ending Balance	867
Natural Gas Distribution [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	746
Goodwill, Acquired During Period	0	[1]
Goodwill, Ending Balance	746
Energy Services [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	105	[2]
Goodwill, Acquired During Period	5	[1]
Goodwill, Ending Balance	110	[2]
Goodwill, Impaired, Accumulated Impairment Loss	252
Other Operations [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	11
Goodwill, Acquired During Period	0	[1]
Goodwill, Ending Balance	$ 11

[1] See Note 3.

[2] Amount presented is net of the accumulated goodwill impairment charge of $252 million recorded in 2012.