Equity (Details) - Reclassifications Out of Accumulated Other Comprehensive Loss - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance expense	$ (204)	$ (193)	$ (662)	$ (693)
Interest expense (interest rate contracts)	(42)	(44)	(128)	(135)
Income before income taxes	19	37	411	682
Income tax benefit	(7)	(14)	(150)	(254)
Cash flow hedges, net of income tax	(12)	8	(261)	(348)
Less noncontrolling Interest	(1)	0	(15)	(14)
Net income (loss) attributable to AGL Resources	11	(8)	246	334
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss) attributable to AGL Resources	(4)	(2)	(14)	(1)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	(1)	0	(5)	6
Income tax benefit	0	0	0	(1)
Cash flow hedges, net of income tax	1	0	5	(5)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedges Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss) attributable to AGL Resources	(2)	0	(5)	5
Reclassification out of Accumulated Other Comprehensive Income [Member] | Retirement Benefit Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Less noncontrolling Interest	1	0	0	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance expense	(6)	(4)	(17)	(12)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Prior Service Cost [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance expense	2	0	2	1
Reclassification out of Accumulated Other Comprehensive Income [Member] | Retirement Benefit Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	(4)	(4)	(15)	(11)
Income tax benefit	2	2	6	5
Net income (loss) attributable to AGL Resources	(2)	(2)	(9)	(6)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Natural Gas Derivatives [Member] | Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cost of goods sold (natural gas contracts)	(2)	(1)	(6)	4
Operation and maintenance expense	0	1	(1)	2
Reclassification out of Accumulated Other Comprehensive Income [Member] | Interest Rate Contract [Member] | Cash Flow Hedges Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense (interest rate contracts)	$ 1	$ 0	$ 2	$ 0