Acquisition (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jan. 03, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Goodwill		$ 867		$ 867		$ 862
Atmos Energy Marketing [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Total purchase price consideration	$ 147
Cash	15
Receivables	140
Natural gas inventory	78
Derivative assets	35
Prepaid expenses and other current assets	5
Property and equipment	8
Identifiable intangibles	25
Total assets acquired	306
Accounts payable	113
Derivative liabilities	43
Other current liabilities	7
Other liabilities	1
Total liabilities assumed	164
Identifiable net assets acquired	142
Goodwill	5
Net assets acquired	147
Amortization of intangible assets		1		1
Business Acquisition, Pro Forma Information [Abstract]
Revenue of acquiree since acquisition date		319		678
Operating income of acquiree since acquisition date		8		25
Operating Revenue		1,387	$ 1,006	3,480	$ 2,585
Net Income		$ 54	$ 11	$ 201	$ 130
Customer Relationships [Member] | Atmos Energy Marketing [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Intangibles estimated fair value	$ 25
Intangibles estimated useful life	15 years