Subsequent Events (Details) $ / shares in Units, $ in Millions			12 Months Ended
	Feb. 10, 2017 USD ($) $ / shares	Jan. 03, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Subsequent Event [Line Items]
Total purchase price consideration			$ 102	$ 0	$ 0
Proceeds from Equity Method Investment, Dividends or Distributions			297	294	305
Enable Midstream Partners [Member]
Subsequent Event [Line Items]
Proceeds from Equity Method Investment, Dividends or Distributions			$ 297	$ 294	$ 298
Subsequent Event [Member] | Enable Midstream Partners [Member]
Subsequent Event [Line Items]
Dividends Payable, Date Declared	Feb. 10, 2017
Common Stock, Dividends, Per Share, Declared | $ / shares	$ 0.318
Proceeds from Equity Method Investment, Dividends or Distributions	$ 74
Subsequent Event [Member] | Energy Services [Member]
Subsequent Event [Line Items]
Number of States in which Entity Operates		33
Number of States in which Entity Operates, No Prior Commercial or Industrial Customers		7
Number of States in which Entity Operates, Prior Other Operations		5
Subsequent Event [Member] | Atmos Energy Marketing [Member]
Subsequent Event [Line Items]
Business Acquisition, Effective Date of Acquisition		Jan. 03, 2017
Total purchase price consideration		$ 140
Payments To Acquire Businesses, Adjustment For Working Capital		$ 100