Note 6 - Debt and Credit Facilities (Details) - Debt Schedule (USD $) In Millions, unless otherwise specified																						6 Months Ended																																																							6 Months Ended
	Jun. 30, 2013		Dec. 31, 2012	Jun. 30, 2012		Jun. 30, 2013 Commercial Paper [Member] AGL Capital [Member]		Dec. 31, 2012 Commercial Paper [Member] AGL Capital [Member]		Jun. 30, 2012 Commercial Paper [Member] AGL Capital [Member]		Jun. 30, 2013 Commercial Paper [Member] Nicor Gas [Member]		Dec. 31, 2012 Commercial Paper [Member] Nicor Gas [Member]	Jun. 30, 2012 Commercial Paper [Member] Nicor Gas [Member]		Jun. 30, 2013 Commercial Paper [Member]		Dec. 31, 2012 Commercial Paper [Member]	Jun. 30, 2012 Commercial Paper [Member]		Jun. 30, 2013 Commercial Paper [Member] Minimum [Member] AGL Capital [Member]		Jun. 30, 2013 Commercial Paper [Member] Minimum [Member] Nicor Gas [Member]	Jun. 30, 2013 Current Portion Long-Term Debt [Member]		Dec. 31, 2012 Current Portion Long-Term Debt [Member]	Jun. 30, 2012 Current Portion Long-Term Debt [Member]		Jun. 30, 2013 Current Portion of Capital Leases [Member]		Dec. 31, 2012 Current Portion of Capital Leases [Member]		Jun. 30, 2012 Current Portion of Capital Leases [Member]		Jun. 30, 2013 Senior Notes [Member]		Dec. 31, 2012 Senior Notes [Member]	Jun. 30, 2012 Senior Notes [Member]		Jun. 30, 2013 First Mortgage Bonds [Member]		Dec. 31, 2012 First Mortgage Bonds [Member]	Jun. 30, 2012 First Mortgage Bonds [Member]		Jun. 30, 2013 Gas Facility Revenue Bonds [Member]		Dec. 31, 2012 Gas Facility Revenue Bonds [Member]	Jun. 30, 2012 Gas Facility Revenue Bonds [Member]		Jun. 30, 2013 Medium-term Notes [Member]		Dec. 31, 2012 Medium-term Notes [Member]	Jun. 30, 2012 Medium-term Notes [Member]		Jun. 30, 2013 Long-Term Debt Principal [Member]		Dec. 31, 2012 Long-Term Debt Principal [Member]	Jun. 30, 2012 Long-Term Debt Principal [Member]		Jun. 30, 2013 Long-Term Debt Fair Value Adjustment [Member]		Dec. 31, 2012 Long-Term Debt Fair Value Adjustment [Member]		Jun. 30, 2012 Long-Term Debt Fair Value Adjustment [Member]		Jun. 30, 2013 Unamortized Debt Premium Discount Net [Member]		Dec. 31, 2012 Unamortized Debt Premium Discount Net [Member]	Jun. 30, 2012 Unamortized Debt Premium Discount Net [Member]		Jun. 30, 2013 Long-Term Debt Non-Principal [Member]		Dec. 31, 2012 Long-Term Debt Non-Principal [Member]	Jun. 30, 2012 Long-Term Debt Non-Principal [Member]		Jun. 30, 2013 Minimum [Member] Current Portion Long-Term Debt [Member]	Jun. 30, 2013 Minimum [Member] Current Portion of Capital Leases [Member]	Jun. 30, 2013 Minimum [Member] Senior Notes [Member]	Jun. 30, 2013 Minimum [Member] First Mortgage Bonds [Member]	Jun. 30, 2013 Minimum [Member] Gas Facility Revenue Bonds [Member]	Jun. 30, 2013 Minimum [Member] Medium-term Notes [Member]	Jun. 30, 2013 Minimum [Member] Long-Term Debt Fair Value Adjustment [Member]		Jun. 30, 2013 Minimum [Member] Unamortized Debt Premium Discount Net [Member]	Jun. 30, 2013 Maximum [Member] Senior Notes [Member]	Jun. 30, 2013 Maximum [Member] First Mortgage Bonds [Member]	Jun. 30, 2013 Maximum [Member] Gas Facility Revenue Bonds [Member]	Jun. 30, 2013 Maximum [Member] Medium-term Notes [Member]	Jun. 30, 2013 Maximum [Member] Long-Term Debt Fair Value Adjustment [Member]
Short-term debt
Maturity Date																						2013	[1]	2013																																																					2013	2013	2015	2016	2022	2017	2016	[2]	n/a	2043	2038	2033	2027	2038	[2]
Weighted average interest rate	4.50%	[3]		4.70%	[3]	0.50%	[1],[3]			0.50%	[1],[3]	0.40%	[3]		0.50%	[3]	0.50%	[3]		0.50%	[3]				4.50%	[3]		4.70%	[3]	5.00%	[3]			4.90%	[3]
Outstanding			$ 226	$ 231		$ 521	[1]	$ 1,063	[1]	$ 731	[1]			$ 314			$ 521		$ 1,377	$ 731							$ 225	$ 230				$ 1	[3]	$ 1
Long-term debt - excluding current portion
Maturity Date																						2013	[1]	2013																																																					2013	2013	2015	2016	2022	2017	2016	[2]	n/a	2043	2038	2033	2027	2038	[2]
Weighted average interest rate																																				5.10%	[3]		5.10%	[3]	5.60%	[3]		5.60%	[3]	0.50%	[3]		1.20%	[3]	7.80%	[3]		7.80%	[3]	4.90%	[3]		5.00%	[3]		[2],[3]				[2],[3]		[3]			[3]		[3]			[3]
Outstanding	3,819		3,327	3,334																																2,825		2,325	2,325		500		500	500		200		200	200		181		181	181		3,706		3,206	3,206		97	[2]	103	[2]	110	[2]	16		18	18		113		121	128
Total debt	$ 4,340		$ 4,930	$ 4,296

[1] As of June 30, 2013, the weighted average interest rate on AGL Capital's commercial paper borrowings was 0.4%.

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

[3] Interest rates are calculated based on the daily weighted average balance outstanding for the six months ended June 30.