Condensed Consolidated Statements of Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Operating revenues (includes revenue taxes of $17 and $57 for the three and six months ended June 30, 2016, respectively, and $18 and $74 for the three and six months ended June 30, 2015, respectively)	$ 571	$ 674	$ 1,905	$ 2,395
Operating expenses
Cost of goods sold	191	222	769	1,157
Operation and maintenance	213	209	454	458
Depreciation and amortization	104	98	206	195
Taxes other than income taxes	37	38	99	114
Merger-related expenses	53	0	56	0
Total operating expenses	598	567	1,584	1,924
Operating (loss) income	(27)	107	321	471
Other income	3	4	6	7
Interest expense, net	(48)	(42)	(95)	(86)
(Loss) income before income taxes	(72)	69	232	392
Income tax (benefit) expense	(24)	25	87	143
Net (loss) income	(48)	44	145	249
Less net income attributable to noncontrolling interest	3	2	14	14
Net (loss) income attributable to Southern Company Gas	$ (51)	$ 42	$ 131	$ 235
Per common share information 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
Cash dividends declared per common share (in dollars per share)	$ 0.53	$ 0.51	$ 1.06	$ 1.02
Weighted average number of common shares outstanding
Basic (in shares)	120.3	119.5	120.2	119.4
Diluted (in shares)	120.5	119.8	120.5	119.7