Significant Accounting Policies and Methods of Application (Details) - Potentially Dilutive Common Share Calculation - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Potentially Dilutive Common Share Calculation [Abstract]
Income from continuing operations attributable to AGL Resources	$ 11	$ 23	$ 246	$ 414
Loss from discontinued operations, net of tax	0	(31)	0	(80)
Net income (loss) attributable to AGL Resources	$ 11	$ (8)	$ 246	$ 334
Denominator:
Basic weighted average number of shares outstanding (in shares)	119.6	119.0	119.5	118.8
Effect of dilutive securities (in shares)	0.4	0.4	0.3	0.4
Diluted weighted average number of shares outstanding (in shares)	120.0	119.4	119.8	119.2
Basic earnings (loss) per common share
Continuing operations (in dollars per share)	$ 0.09	$ 0.19	$ 2.06	$ 3.48
Discontinued operations (in dollars per share)	0.00	(0.25)	0.00	(0.67)
Basic earnings (loss) per common share attributable to AGL Resources Inc. (in dollars per share)	0.09	(0.06)	2.06	2.81
Diluted earnings (loss) per common share
Continuing operations (in dollars per share)	0.09	0.19	2.05	3.47
Discontinued operations (in dollars per share)	0.00	(0.25)	0.00	(0.67)
Diluted earnings (loss) per common share attributable to AGL Resources Inc. (in dollars per share)	$ 0.09	$ (0.06)	$ 2.05	$ 2.80