Note 6 - Debt and Credit Facilities (Details) - Debt Schedule (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
Short-term debt
Weighted average interest rate		[1]			4.60%	[1]
Outstanding			$ 226		$ 226
Long-term debt - excluding current portion
Outstanding	3,816		3,327		3,330
Total debt	4,648		4,930		4,604
Commercial Paper [Member] | AGL Capital [Member]
Short-term debt
Maturity Date	2013	[2]
Weighted average interest rate	0.40%	[1],[2]			0.50%	[1],[2]
Outstanding	680	[2]	1,063	[2]	875	[2]
Long-term debt - excluding current portion
Maturity Date	2013	[2]
Commercial Paper [Member] | Nicor Gas [Member]
Short-term debt
Maturity Date	2013
Weighted average interest rate	0.30%	[1]			0.50%	[1]
Outstanding	152		314		173
Long-term debt - excluding current portion
Maturity Date	2013
Commercial Paper [Member]
Short-term debt
Weighted average interest rate	0.40%	[1]			0.50%	[1]
Outstanding	832		1,377		1,048
Current Portion Long-Term Debt [Member]
Short-term debt
Maturity Date	n/a
Weighted average interest rate		[1]			4.60%	[1]
Outstanding			225		225
Long-term debt - excluding current portion
Maturity Date	n/a
Current Portion of Capital Leases [Member]
Short-term debt
Maturity Date	n/a
Weighted average interest rate		[1]			4.90%	[1]
Outstanding			1	[1]	1
Long-term debt - excluding current portion
Maturity Date	n/a
Senior Notes [Member]
Short-term debt
Maturity Date	-	[1]
Long-term debt - excluding current portion
Maturity Date	-	[1]
Weighted average interest rate	5.10%	[1]			5.10%	[1]
Outstanding	2,825	[1]	2,325		2,325
First Mortgage Bonds [Member]
Short-term debt
Maturity Date	-	[1]
Long-term debt - excluding current portion
Maturity Date	-	[1]
Weighted average interest rate	5.60%	[1]			5.60%	[1]
Outstanding	500	[1]	500		500
Gas Facility Revenue Bonds [Member]
Short-term debt
Maturity Date	-	[1]
Long-term debt - excluding current portion
Maturity Date	-	[1]
Weighted average interest rate	0.80%	[1]			1.10%	[1]
Outstanding	200	[1]	200		200
Medium-term Notes [Member]
Short-term debt
Maturity Date	-	[1]
Long-term debt - excluding current portion
Maturity Date	-	[1]
Weighted average interest rate	7.80%	[1]			7.80%	[1]
Outstanding	181	[1]	181		181
Long-Term Debt Principal [Member]
Long-term debt - excluding current portion
Weighted average interest rate	4.90%	[1]			5.00%	[1]
Outstanding	3,706		3,206		3,206
Long-Term Debt Fair Value Adjustment [Member]
Short-term debt
Maturity Date	-	[1],[3]
Long-term debt - excluding current portion
Maturity Date	-	[1],[3]
Weighted average interest rate		[1],[3]				[1],[3]
Outstanding	94	[1],[3]	103	[3]	106	[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	16		18		18
Long-Term Debt Non-Principal [Member]
Long-term debt - excluding current portion
Weighted average interest rate		[1]				[1]
Outstanding	$ 110		$ 121		$ 124
Minimum [Member] | Senior Notes [Member]
Short-term debt
Maturity Date	2015
Long-term debt - excluding current portion
Maturity Date	2015
Minimum [Member] | First Mortgage Bonds [Member]
Short-term debt
Maturity Date	2016
Long-term debt - excluding current portion
Maturity Date	2016
Minimum [Member] | Gas Facility Revenue Bonds [Member]
Short-term debt
Maturity Date	2022
Long-term debt - excluding current portion
Maturity Date	2022
Minimum [Member] | Medium-term Notes [Member]
Short-term debt
Maturity Date	2017
Long-term debt - excluding current portion
Maturity Date	2017
Minimum [Member] | Long-Term Debt Fair Value Adjustment [Member]
Short-term debt
Maturity Date	2016	[3]
Long-term debt - excluding current portion
Maturity Date	2016	[3]
Maximum [Member] | Senior Notes [Member]
Short-term debt
Maturity Date	2043
Long-term debt - excluding current portion
Maturity Date	2043
Maximum [Member] | First Mortgage Bonds [Member]
Short-term debt
Maturity Date	2038
Long-term debt - excluding current portion
Maturity Date	2038
Maximum [Member] | Gas Facility Revenue Bonds [Member]
Short-term debt
Maturity Date	2033
Long-term debt - excluding current portion
Maturity Date	2033
Maximum [Member] | Medium-term Notes [Member]
Short-term debt
Maturity Date	2027
Long-term debt - excluding current portion
Maturity Date	2027
Maximum [Member] | Long-Term Debt Fair Value Adjustment [Member]
Short-term debt
Maturity Date	2038	[3]
Long-term debt - excluding current portion
Maturity Date	2038	[3]

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

[2] As of September 30, 2013, the effective interest rates on our commercial paper borrowings were 0.4% for AGL Capital and 0.2% for Nicor Gas.

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