Note 3 - Merger with Nicor (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified		12 Months Ended
	Dec. 09, 2011	Dec. 31, 2012 Merger Transaction Costs [Member] Before Tax [Member]	Dec. 31, 2011 Merger Transaction Costs [Member] Before Tax [Member]	Dec. 31, 2010 Merger Transaction Costs [Member] Before Tax [Member]	Dec. 31, 2012 Merger Transaction Costs [Member] After Tax [Member]	Dec. 31, 2011 Merger Transaction Costs [Member] After Tax [Member]	Dec. 31, 2010 Merger Transaction Costs [Member] After Tax [Member]	Dec. 31, 2011 Change in Control and Other Benefit Payments [Member] Before Tax [Member]	Dec. 31, 2011 Change in Control and Other Benefit Payments [Member] After Tax [Member]	Dec. 31, 2011 Debt Issuance Costs and Interest Expense [Member] Before Tax [Member]	Dec. 31, 2011 Debt Issuance Costs and Interest Expense [Member] After Tax [Member]	Dec. 31, 2011 Change in Control Expenses [Member] After Tax [Member]	Dec. 31, 2012 Nicor Gas [Member]	Dec. 31, 2011 AGL Resources Inc. and Nicor Inc. [Member]	Dec. 31, 2010 AGL Resources Inc. and Nicor Inc. [Member]
Stock Consideration Paid Per Share Of Acquirees Stock	0.8382
(in Dollars per share)	$ 21.20
Nicors Shareholders Ownership Percentage Of The Combined Company	33.00%
Liabilities, Fair Value Adjustment													$ 99
Assets, Fair Value Adjustment													35
Business Combination, Separately Recognized Transactions, Expenses and Losses Recognized		$ 20	$ 57	$ 6	$ 13	$ 48	$ 4	$ 34	$ 31	$ 25	$ 16	$ 31		$ 96	$ 27