Note 8 - Debt and Credit Facilities (Detail) - Debt Schedule (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Short-term debt
Weighted average interest rate	4.60%	[1]	8.00%	[1]
Outstanding	$ 226		$ 17
Long-term debt - excluding current portion
Outstanding	3,431
Total long-term debt	3,327		3,561
Total debt	4,930		4,899
Commercial Paper [Member] | AGL Capital [Member]
Short-term debt
Year(s) due	2013	[2]
Weighted average interest rate	0.50%	[1],[2]	0.40%	[1],[2]
Outstanding	1,063	[2]	869	[2]
Long-term debt - excluding current portion
Year(s) due	2013	[2]
Commercial Paper [Member] | Nicor Gas [Member]
Short-term debt
Year(s) due	2013	[2]
Weighted average interest rate	0.40%	[1],[2]	0.40%	[1],[2]
Outstanding	314	[2]	452	[2]
Long-term debt - excluding current portion
Year(s) due	2013	[2]
Commercial Paper [Member]
Short-term debt
Weighted average interest rate	0.50%	[1]	0.40%	[1]
Outstanding	1,377		1,321
Current Portion Long-Term Debt [Member]
Short-term debt
Year(s) due	2013
Weighted average interest rate	4.60%	[1]	8.30%	[1]
Outstanding	225		15
Long-term debt - excluding current portion
Year(s) due	2013
Current Portion of Capital Leases [Member]
Short-term debt
Year(s) due	2013
Weighted average interest rate	4.90%	[1]	4.90%	[1]
Outstanding	1		2
Long-term debt - excluding current portion
Year(s) due	2013
Senior Notes [Member]
Short-term debt
Year(s) due	-
Long-term debt - excluding current portion
Year(s) due	2015
Year(s) due	-
Year(s) due	2041
Weighted average interest rate	5.10%	[1]	5.40%	[1]
Outstanding	2,325		2,550
First Mortgage Bonds [Member]
Short-term debt
Year(s) due	-
Long-term debt - excluding current portion
Year(s) due	2016
Year(s) due	-
Year(s) due	2038
Weighted average interest rate	5.60%	[1]	5.60%	[1]
Outstanding	500		500
Gas Facility Revenue Bonds [Member]
Short-term debt
Year(s) due	-
Long-term debt - excluding current portion
Year(s) due	2022
Year(s) due	-
Year(s) due	2033
Weighted average interest rate	1.20%	[1]	1.20%	[1]
Outstanding	200		200
Medium-term Notes [Member]
Short-term debt
Year(s) due	-
Long-term debt - excluding current portion
Year(s) due	2017
Year(s) due	-
Year(s) due	2027
Weighted average interest rate	7.80%	[1]	7.80%	[1]
Outstanding	181		181
Long-Term Debt Principal [Member]
Long-term debt - excluding current portion
Weighted average interest rate	5.00%	[1]	5.70%	[1]
Outstanding	3,206		3,431
Fair Value Adjustment [Member]
Short-term debt
Year(s) due	-	[3]
Long-term debt - excluding current portion
Year(s) due	2016	[3]
Year(s) due	-	[3]
Year(s) due	2038	[3]
Weighted average interest rate		[1],[3]		[1],[3]
Outstanding	103	[3]	112	[3]
Unamortized Debt Premium Discount Net [Member]
Short-term debt
Year(s) due	n/a
Long-term debt - excluding current portion
Year(s) due	n/a
Weighted average interest rate		[1]		[1]
Outstanding	18		18
Long-Term Debt Non-Principal [Member]
Long-term debt - excluding current portion
Weighted average interest rate		[1]		[1]
Outstanding	$ 121		$ 130

[1] Interest rates are calculated based on the daily weighted average balance for the applicable category outstanding for the twelve months ended December 31, 2012 and 2011.

[2] As of December 31, 2012, the weighted average interest rates on our commercial paper borrowings were 0.5% for AGL Capital and 0.4% for Nicor Gas.

[3] See Note 4 for additional information on our fair value measurements.