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Note 8 - Debt and Credit Facilities (Details) - Debt Schedule (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Short-term debt    
Weighted average interest rate    [1] 4.60% [1]
Outstanding   $ 226,000,000
Long-term debt - excluding current portion    
Outstanding 3,813,000,000 3,327,000,000
Total debt 4,984 4,930
Commercial Paper [Member] | Minimum [Member] | AGL Capital [Member]
    
Short-term debt    
Maturity Date    [2]  
Long-term debt - excluding current portion    
Maturity Date    [2]  
Commercial Paper [Member] | Minimum [Member] | Nicor Gas [Member]
    
Short-term debt    
Maturity Date    [2]  
Long-term debt - excluding current portion    
Maturity Date    [2]  
Commercial Paper [Member] | Maximum [Member] | AGL Capital [Member]
    
Short-term debt    
Maturity Date    [2]  
Long-term debt - excluding current portion    
Maturity Date    [2]  
Commercial Paper [Member] | Maximum [Member] | Nicor Gas [Member]
    
Short-term debt    
Maturity Date    [2]  
Long-term debt - excluding current portion    
Maturity Date    [2]  
Commercial Paper [Member] | AGL Capital [Member]
    
Short-term debt    
Maturity Date 2014 [2]  
Weighted average interest rate 0.40% [1],[2] 0.50% [1],[2]
Outstanding 857,000,000 [2] 1,063,000,000 [2]
Long-term debt - excluding current portion    
Maturity Date 2014 [2]  
Commercial Paper [Member] | Nicor Gas [Member]
    
Short-term debt    
Maturity Date 2014 [2]  
Weighted average interest rate 0.30% [1],[2] 0.40% [1],[2]
Outstanding 314,000,000 [2] 314,000,000 [2]
Long-term debt - excluding current portion    
Maturity Date 2014 [2]  
Commercial Paper [Member]
    
Short-term debt    
Weighted average interest rate 0.40% [1] 0.50% [1]
Outstanding 1,171,000,000 1,377,000,000
Current Portion Long-Term Debt [Member]
    
Short-term debt    
Maturity Date     
Weighted average interest rate    [1] 4.60% [1]
Outstanding   225,000,000
Long-term debt - excluding current portion    
Maturity Date     
Current Portion of Capital Leases [Member]
    
Short-term debt    
Maturity Date     
Weighted average interest rate    [1] 4.90% [1]
Outstanding   1,000,000
Long-term debt - excluding current portion    
Maturity Date     
Senior Notes [Member] | Minimum [Member]
    
Short-term debt    
Maturity Date 2015  
Long-term debt - excluding current portion    
Maturity Date 2015  
Senior Notes [Member] | Maximum [Member]
    
Short-term debt    
Maturity Date 2043  
Long-term debt - excluding current portion    
Maturity Date 2043  
Senior Notes [Member]
    
Long-term debt - excluding current portion    
Weighted average interest rate 5.00% [1] 5.10% [1]
Outstanding 2,825,000,000 2,325,000,000
First Mortgage Bonds [Member] | Minimum [Member]
    
Short-term debt    
Maturity Date 2016  
Long-term debt - excluding current portion    
Maturity Date 2016  
First Mortgage Bonds [Member] | Maximum [Member]
    
Short-term debt    
Maturity Date 2038  
Long-term debt - excluding current portion    
Maturity Date 2038  
First Mortgage Bonds [Member]
    
Long-term debt - excluding current portion    
Weighted average interest rate 5.60% [1] 5.60% [1]
Outstanding 500,000,000 500,000,000
Gas Facility Revenue Bonds [Member] | Minimum [Member]
    
Short-term debt    
Maturity Date 2022  
Long-term debt - excluding current portion    
Maturity Date 2022  
Gas Facility Revenue Bonds [Member] | Maximum [Member]
    
Short-term debt    
Maturity Date 2033  
Long-term debt - excluding current portion    
Maturity Date 2033  
Gas Facility Revenue Bonds [Member]
    
Long-term debt - excluding current portion    
Weighted average interest rate 1.00% [1] 1.20% [1]
Outstanding 200,000,000 200,000,000
Medium-term Notes [Member] | Minimum [Member]
    
Short-term debt    
Maturity Date 2017  
Long-term debt - excluding current portion    
Maturity Date 2017  
Medium-term Notes [Member] | Maximum [Member]
    
Short-term debt    
Maturity Date 2027  
Long-term debt - excluding current portion    
Maturity Date 2027  
Medium-term Notes [Member]
    
Long-term debt - excluding current portion    
Weighted average interest rate 7.80% [1] 7.80% [1]
Outstanding 181,000,000 181,000,000
Long-Term Debt Principal [Member]
    
Long-term debt - excluding current portion    
Weighted average interest rate 4.90% [1] 5.00% [1]
Outstanding 3,706,000,000 3,206,000,000
Long-Term Debt Fair Value Adjustment [Member] | Minimum [Member]
    
Short-term debt    
Maturity Date 2016 [3]  
Long-term debt - excluding current portion    
Maturity Date 2016 [3]  
Long-Term Debt Fair Value Adjustment [Member] | Maximum [Member]
    
Short-term debt    
Maturity Date 2038 [3]  
Long-term debt - excluding current portion    
Maturity Date 2038 [3]  
Long-Term Debt Fair Value Adjustment [Member]
    
Short-term debt    
Maturity Date    [3]  
Long-term debt - excluding current portion    
Maturity Date    [3]  
Weighted average interest rate    [1],[3]    [1],[3]
Outstanding 91,000,000 [3] 103,000,000 [3]
Unamortized Debt Premium Discount Net [Member]
    
Short-term debt    
Maturity Date     
Long-term debt - excluding current portion    
Maturity Date     
Weighted average interest rate    [1]    [1]
Outstanding 16,000,000 18,000,000
Long-Term Debt Non-Principal [Member]
    
Long-term debt - excluding current portion    
Weighted average interest rate    [1]    [1]
Outstanding $ 107,000,000 $ 121,000,000
[1] Interest rates are calculated based on the daily weighted average balance outstanding for the 12 months ended December 31, 2013 and 2012.
[2] As of December 31, 2013, the effective interest rates on our commercial paper borrowings were 0.4% for AGL Capital and 0.3% for Nicor Gas.
[3] See Note 4 for additional information on our fair value measurements.