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Debt and Credit Facilities (Details) - Debt Schedule - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Sep. 30, 2014
Long-term debt - excluding current portion      
Unamortized debt premium, net $ 16 $ 16 $ 16
Unamortized debt issuance costs (18) (21) (21)
Total long-term debt - excluding current portion 3,150 3,581 3,584
Total debt $ 4,461 4,956 $ 4,465
Current Portion Long-Term Debt [Member]      
Short-term debt      
Maturity Date 2016    
Weighted average interest rate 5.90%   5.00%
Outstanding $ 425 200 $ 200
Long-term debt - excluding current portion      
Maturity Date 2016    
Senior Notes [Member]      
Long-term debt - excluding current portion      
Weighted average interest rate 4.80%   5.00%
Outstanding $ 2,325 2,625 $ 2,625
First Mortgage Bonds [Member]      
Long-term debt - excluding current portion      
Weighted average interest rate 5.90%   5.60%
Outstanding $ 375 500 $ 500
Gas Facility Revenue Bonds [Member]      
Long-term debt - excluding current portion      
Weighted average interest rate 0.90%   0.90%
Outstanding $ 200 200 $ 200
Medium-term Notes [Member]      
Long-term debt - excluding current portion      
Weighted average interest rate 7.80%   7.80%
Outstanding $ 181 181 $ 181
Principal Long-term Debt [Member]      
Long-term debt - excluding current portion      
Weighted average interest rate 4.70%   4.90%
Outstanding $ 3,081 3,506 $ 3,506
Long-term Debt Fair Value Adjustment [Member]      
Long-term debt - excluding current portion      
Outstanding 71 80 83
Long-term Debt Non-principal [Member]      
Long-term debt - excluding current portion      
Outstanding $ 69 75 $ 78
Commercial Paper [Member]      
Short-term debt      
Weighted average interest rate 0.40%   0.30%
Outstanding $ 886 1,175 $ 681
Commercial Paper [Member] | AGL Capital [Member]      
Short-term debt      
Maturity Date 2015    
Weighted average interest rate 0.50%   0.30%
Outstanding $ 450 590 $ 292
Long-term debt - excluding current portion      
Maturity Date 2015    
Commercial Paper [Member] | Nicor Gas [Member]      
Short-term debt      
Maturity Date 2015    
Weighted average interest rate 0.40%   0.20%
Outstanding $ 436 $ 585 $ 389
Long-term debt - excluding current portion      
Maturity Date 2015    
Minimum [Member] | Senior Notes [Member]      
Short-term debt      
Maturity Date 2016    
Long-term debt - excluding current portion      
Maturity Date 2016    
Minimum [Member] | First Mortgage Bonds [Member]      
Short-term debt      
Maturity Date 2019    
Long-term debt - excluding current portion      
Maturity Date 2019    
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    
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