Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 12	$ (8)	$ 261	$ 348
Retirement benefit plans
Reclassification of actuarial losses to net benefit cost (net of income tax of $3 and $7 for the three and nine months ended September 30, 2015, respectively, and $2 and $5 for the three and nine months ended September 30, 2014, respectively)	3	2	10	7
Reclassification of prior service credits to net benefit cost (net of income tax of $1 for the three and nine months ended September 30, 2015)	(1)	0	(1)	(1)
Retirement benefit plans, net	2	2	9	6
Cash flow hedges, net of tax
Net derivative instruments gain (loss) arising during the period (net of income tax of $18 and $1 for the three and nine months ended September 30, 2015, respectively, and $0 for the three and nine months ended September 30, 2014)	(30)	(2)	(3)	2
Reclassification of realized derivative instruments (gain) loss to net income (net of income tax of $0 for the three and nine months ended September 30, 2015, and $0 and $1 for the three and nine months ended September 30, 2014, respectively)	1	0	5	(5)
Cash flow hedges, net	(29)	(2)	2	(3)
Other comprehensive (loss) income, net of tax	(27)	0	11	3
Comprehensive (loss) income	(15)	(8)	272	351
Less comprehensive income attributable to noncontrolling interest	0	0	15	14
Comprehensive (loss) income attributable to AGL Resources	$ (15)	$ (8)	$ 257	$ 337