Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (48)	$ 44	$ 145	$ 249
Retirement benefit plans, net of tax
Reclassification of actuarial losses to net benefit cost (net of income tax of $2 and $4 for the three and six months ended June 30, 2016, respectively, and $2 and $4 for the three and six months ended June 30, 2015, respectively)	2	4	5	7
Retirement benefit plans, net	2	4	5	7
Cash flow hedges, net of tax
Net derivative (loss) gain arising during the period (net of income tax of $7 and $23 for the three and six months ended June 30, 2016, respectively, and $16 and $17 for the three and six months ended June 30, 2015, respectively)	(12)	25	(41)	27
Reclassification of realized derivative gain to net income (net of income tax of less than $1 for the three and six months ended June 30, 2016 and 2015)	2	4	1	4
Cash flow hedges, net	(10)	29	(40)	31
Other comprehensive (loss) income, net of tax	(8)	33	(35)	38
Comprehensive (loss) income	(56)	77	110	287
Less comprehensive income attributable to noncontrolling interest	3	3	14	15
Comprehensive (loss) income attributable to Southern Company Gas	$ (59)	$ 74	$ 96	$ 272