Condensed Consolidated Statements of Equity (Unaudited) - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Treasury Shares [Member]	Noncontrolling Interest [Member]
Balance at Dec. 31, 2014	$ 3,828	$ 599	$ 2,087	$ 1,312	$ (206)	$ (8)	$ 44
Balance (in shares) at Dec. 31, 2014		119,600,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Southern Company Gas	249			235			14
Net income attributable to Southern Company Gas	235
Other comprehensive income (loss)	38				37		1
Dividends on common stock	(122)			(122)
Distribution to noncontrolling interest	(18)						(18)
Stock granted, share-based compensation, net of forfeitures	(13)		(13)
Stock issued, dividend reinvestment plan	6	$ 1	5
Stock issued, dividend reinvestment plan (in shares)		100,000
Stock issued, share-based compensation, net of forfeitures	15	$ 1	14
Stock issued, share-based compensation, net of forfeitures (in shares)		400,000
Share-based compensation expense, net of tax	6		6
Balance at Jun. 30, 2015	$ 3,989	$ 601	2,099	1,425	(169)	(8)	41
Balance, shares (in shares) at Jun. 30, 2015	120,081,995	120,100,000
Balance at Mar. 31, 2015					(201)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Southern Company Gas	$ 44
Net income attributable to Southern Company Gas	42
Other comprehensive income (loss)	33				32
Balance at Jun. 30, 2015	$ 3,989	$ 601	2,099	1,425	(169)	(8)	41
Balance, shares (in shares) at Jun. 30, 2015	120,081,995	120,100,000
Balance at Dec. 31, 2015	$ 3,975	$ 603	2,099	1,421	(186)	(8)	46
Balance (in shares) at Dec. 31, 2015	120,376,721	120,400,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Southern Company Gas	$ 145
Net income attributable to Southern Company Gas	131			131
Other comprehensive income (loss)	(35)				(35)
Dividends on common stock	(128)			(128)
Stock granted, share-based compensation, net of forfeitures	(9)		(9)
Stock issued, dividend reinvestment plan	6	$ 0	6
Stock issued, dividend reinvestment plan (in shares)		0
Stock issued, share-based compensation, net of forfeitures	17	$ 2	15
Stock issued, share-based compensation, net of forfeitures (in shares)		300,000
Share-based compensation expense, net of tax	22		22
Reclassification of noncontrolling interest	(46)						(46)
Balance at Jun. 30, 2016	$ 3,933	$ 605	2,133	1,424	(221)	(8)	0
Balance, shares (in shares) at Jun. 30, 2016	120,741,810	120,700,000
Balance at Mar. 31, 2016					(213)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Southern Company Gas	$ (48)
Net income attributable to Southern Company Gas	(51)
Other comprehensive income (loss)	(8)				(8)
Balance at Jun. 30, 2016	$ 3,933	$ 605	$ 2,133	$ 1,424	$ (221)	$ (8)	$ 0
Balance, shares (in shares) at Jun. 30, 2016	120,741,810	120,700,000