Note 7 - Debt and Credit Facilities (Detail) - Schedule of Debt (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2011
Short-term debt
Weighted average interest rate	4.70%	[1]			7.90%	[1]
Outstanding	$ 231		$ 17		$ 12
Long-term debt – excluding current portion
Outstanding	3,334		3,561		2,164
Total debt	4,296		4,899		2,318
Commercial Paper [Member] | AGL Capital [Member]
Short-term debt
Maturity Date Range	2012	[2]
Weighted average interest rate	0.50%	[1],[2]			0.40%	[1],[2]
Outstanding	731	[2]	869	[2]	142	[2]
Long-term debt – excluding current portion
Maturity Date Range	2012	[2]
Commercial Paper [Member] | Nicor Gas [Member]
Short-term debt
Weighted average interest rate	0.50%	[1]
Outstanding	0		452
Commercial Paper [Member]
Short-term debt
Weighted average interest rate	0.50%	[1]			0.40%	[1]
Outstanding	731		1,321		142
Current Portion Long-Term Debt [Member]
Short-term debt
Maturity Date Range	2012
Maturity Date Range	2013
Weighted average interest rate	4.70%	[1]			8.40%	[1]
Outstanding	230		15		10
Long-term debt – excluding current portion
Maturity Date Range	2012
Maturity Date Range	2013
Current Portion Capital Leases [Member]
Short-term debt
Maturity Date Range	2013
Weighted average interest rate	4.90%	[1]			4.90%	[1]
Outstanding	1		2		2
Long-term debt – excluding current portion
Maturity Date Range	2013
Notes Senior [Member]
Short-term debt
Maturity Date Range	2015
Maturity Date Range	2041
Long-term debt – excluding current portion
Maturity Date Range	2015
Maturity Date Range	2041
Weighted average interest rate	5.10%	[1]			5.50%	[1]
Outstanding	2,325		2,550		1,775
First Mortgage Bonds [Member]
Short-term debt
Maturity Date Range	2016
Maturity Date Range	2038
Long-term debt – excluding current portion
Maturity Date Range	2016
Maturity Date Range	2038
Weighted average interest rate	5.60%	[1]
Outstanding	500		500
Gas Facility Revenue Bonds [Member]
Short-term debt
Maturity Date Range	2022
Maturity Date Range	2033
Long-term debt – excluding current portion
Maturity Date Range	2022
Maturity Date Range	2033
Weighted average interest rate	1.20%	[1]			1.20%	[1]
Outstanding	200		200		200
Medium-term Notes [Member]
Short-term debt
Maturity Date Range	2017
Maturity Date Range	2027
Long-term debt – excluding current portion
Maturity Date Range	2017
Maturity Date Range	2027
Weighted average interest rate	7.80%	[1]			7.80%	[1]
Outstanding	181		181		186
Capital Lease Obligations [Member]
Long-term debt – excluding current portion
Weighted average interest rate					4.20%	[1]
Outstanding	0		0		3
Long-Term Debt Principal [Member]
Long-term debt – excluding current portion
Weighted average interest rate	5.00%	[1]			5.00%	[1]
Outstanding	3,206		3,431		2,164
Fair Value Adjustment [Member]
Short-term debt
Maturity Date Range	2016
Maturity Date Range	2038
Long-term debt – excluding current portion
Maturity Date Range	2016
Maturity Date Range	2038
Outstanding	110		112		0
Unamortized Debt Premium Discount Net [Member]
Long-term debt – excluding current portion
Outstanding	18		18
Long-Term Debt Non-Principal [Member]
Long-term debt – excluding current portion
Outstanding	$ 128		$ 130		$ 0

[1] Interest rates are calculated based on the daily average balance outstanding.

[2] Weighted average interest rate was 0.5% as of June 30, 2012 and 0.3% as of June 30, 2011.