Note 7 - Debt and Credit Facilities (Detail) - Debt Schedule (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011
Short-term debt
Weighted average interest rate	4.60%	[1]			8.00%	[1]
Outstanding	$ 226		$ 17		$ 17
Long-term debt - excluding current portion
Outstanding	3,330		3,561		2,687
Total debt	4,604		4,899		2,704
Commercial Paper [Member] | AGL Capital [Member]
Short-term debt
Maturity Date	2012	[2]
Weighted average interest rate	0.50%	[1],[2]			0.40%	[1],[2]
Outstanding	875	[2]	869	[2]	0	[2]
Long-term debt - excluding current portion
Maturity Date	2012	[2]
Commercial Paper [Member] | Nicor Gas [Member]
Short-term debt
Maturity Date	2012	[2]
Weighted average interest rate	0.50%	[1],[2]				[1],[2]
Outstanding	173	[2]	452	[2]		[2]
Long-term debt - excluding current portion
Maturity Date	2012	[2]
Commercial Paper [Member]
Short-term debt
Weighted average interest rate	0.50%	[1]			0.40%	[1]
Outstanding	1,048		1,321		0
Current Portion Long-Term Debt [Member]
Short-term debt
Maturity Date	2013
Weighted average interest rate	4.60%	[1]			8.30%	[1]
Outstanding	225		15		15
Long-term debt - excluding current portion
Maturity Date	2013
Current Portion Capital Leases [Member]
Short-term debt
Maturity Date	2012
Maturity Date	2013
Weighted average interest rate	4.90%	[1]			4.90%	[1]
Outstanding	1		2		2
Long-term debt - excluding current portion
Maturity Date	2012
Maturity Date	2013
Senior Notes [Member]
Short-term debt
Maturity Date	2015
Maturity Date	2041
Long-term debt - excluding current portion
Maturity Date	2015
Maturity Date	2041
Weighted average interest rate	5.10%	[1]			5.50%	[1]
Outstanding	2,325		2,550		2,275
First Mortgage Bonds [Member]
Short-term debt
Maturity Date	2016
Maturity Date	2038
Long-term debt - excluding current portion
Maturity Date	2016
Maturity Date	2038
Weighted average interest rate	5.60%	[1]				[1]
Outstanding	500		500
Gas Facility Revenue Bonds [Member]
Short-term debt
Maturity Date	2022
Maturity Date	2033
Long-term debt - excluding current portion
Maturity Date	2022
Maturity Date	2033
Weighted average interest rate	1.10%	[1]			1.10%	[1]
Outstanding	200		200		200
Medium-term Notes [Member]
Short-term debt
Maturity Date	2017
Maturity Date	2027
Long-term debt - excluding current portion
Maturity Date	2017
Maturity Date	2027
Weighted average interest rate	7.80%	[1]			7.80%	[1]
Outstanding	181		181		181
Long-Term Debt Principal [Member]
Long-term debt - excluding current portion
Weighted average interest rate	5.00%	[1]			5.30%	[1]
Outstanding	3,206		3,431		2,656
Fair Value Adjustment [Member]
Short-term debt
Maturity Date	2016	[3]
Maturity Date	2038	[3]
Long-term debt - excluding current portion
Maturity Date	2016	[3]
Maturity Date	2038	[3]
Weighted average interest rate		[1],[3]				[1],[3]
Outstanding	106	[3]	112	[3]	11	[3]
Unamortized Debt Premium Discount Net [Member]
Short-term debt
Maturity Date	n/a
Long-term debt - excluding current portion
Maturity Date	n/a
Weighted average interest rate		[1]				[1]
Outstanding	18		18		20
Long-Term Debt Non-Principal [Member]
Long-term debt - excluding current portion
Weighted average interest rate		[1]				[1]
Outstanding	$ 124		$ 130		$ 31

[1] Interest rates are calculated based on the daily weighted average balance for the applicable category outstanding for the nine months ended September 30.

[2] As of September 30, 2012, the weighted average interest rates on our commercial paper borrowings were 0.5% for AGL Capital and 0.4% for Nicor Gas.

[3] Fair value adjustment of the first mortgage bonds relates to the step up to fair value of the long-term debt assumed in the Nicor merger and was $92 million as of September 30, 2012 and $99 million as of December 31, 2011. Fair value adjustment related to interest rate hedges was $14 million as of September 30, 2012, $13 million as of December 31, 2011 and $11 million as of September 30, 2011.