Significant Accounting Policies and Methods of Application - Potentially Dilutive Common Share Calculation (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Potentially Dilutive Common Share Calculation [Abstract]
Net (loss) income attributable to Southern Company Gas	$ (51)	$ 42	$ 131	$ 235
Denominator
Basic weighted average number of shares outstanding (in shares)	120.3	119.5	120.2	119.4
Effect of dilutive securities (in shares)	0.2	0.3	0.3	0.3
Diluted weighted average number of shares outstanding (in shares)	120.5	119.8	120.5	119.7
(Loss) earnings per common share attributable to Southern Company Gas
Basic (loss) earnings per common share (in dollars per share)	$ (0.43)	$ 0.35	$ 1.09	$ 1.97
Diluted (loss) earnings per common share (in dollars per share)	$ (0.43)	$ 0.35	$ 1.09	$ 1.96