Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 373	$ 500	$ 313
Retirement benefit plans, net of tax
Actuarial (loss) gain arising during the period (net of income tax of $0, $48 and $46)	0	(71)	66
Reclassification of actuarial loss to net benefit cost (net of income tax of $9, $6 and $10)	14	9	15
Reclassification of prior service cost to net benefit cost (net of income tax of $0, $1 and $2)	(2)	(1)	(3)
Retirement benefit plans, net	12	(63)	78
Cash flow hedges, net of tax
Net derivative instrument (loss) gain arising during the period (net of income tax of $3, $2 and $1)	0	(6)	1
Reclassification of realized derivative loss (gain) to net income (net of income tax of $1, $2 and $1)	8	(3)	3
Cash flow hedges, net	8	(9)	4
Other comprehensive income (loss), net of tax	20	(72)	82
Comprehensive income	393	428	395
Less comprehensive income attributable to noncontrolling interest	20	16	18
Comprehensive income attributable to AGL Resources	$ 373	$ 412	$ 377