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Note 8 - Debt and Credit Facilities (Details) - Debt Schedule (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Long-term debt - excluding current portion    
Outstanding $ 3,706us-gaap_LongTermDebt  
Total long-term debt 3,602us-gaap_LongTermDebtAndCapitalLeaseObligations 3,813us-gaap_LongTermDebtAndCapitalLeaseObligations
Total debt 4,977us-gaap_DebtLongtermAndShorttermCombinedAmount 4,984us-gaap_DebtLongtermAndShorttermCombinedAmount
Commercial Paper [Member] | Minimum [Member] | AGL Capital [Member]    
Short-term debt    
Maturity Date    [1]  
Long-term debt - excluding current portion    
Maturity Date    [1]  
Commercial Paper [Member] | Minimum [Member] | Nicor Gas [Member]    
Short-term debt    
Maturity Date    [1]  
Long-term debt - excluding current portion    
Maturity Date    [1]  
Commercial Paper [Member] | Maximum [Member] | AGL Capital [Member]    
Short-term debt    
Maturity Date    [1]  
Long-term debt - excluding current portion    
Maturity Date    [1]  
Commercial Paper [Member] | Maximum [Member] | Nicor Gas [Member]    
Short-term debt    
Maturity Date    [1]  
Long-term debt - excluding current portion    
Maturity Date    [1]  
Commercial Paper [Member] | AGL Capital [Member]    
Short-term debt    
Maturity Date 2015 [1]  
Weighted average interest rate 0.30%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= gas_AGLCapitalMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [1],[2] 0.40%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= gas_AGLCapitalMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [1],[2]
Outstanding 590us-gaap_DebtCurrent
/ dei_LegalEntityAxis
= gas_AGLCapitalMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [1] 857us-gaap_DebtCurrent
/ dei_LegalEntityAxis
= gas_AGLCapitalMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [1]
Long-term debt - excluding current portion    
Maturity Date 2015 [1]  
Commercial Paper [Member] | Nicor Gas [Member]    
Short-term debt    
Maturity Date 2015 [1]  
Weighted average interest rate 0.20%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= gas_NicorGasMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [1],[2] 0.30%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= gas_NicorGasMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [1],[2]
Outstanding 585us-gaap_DebtCurrent
/ dei_LegalEntityAxis
= gas_NicorGasMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [1] 314us-gaap_DebtCurrent
/ dei_LegalEntityAxis
= gas_NicorGasMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [1]
Long-term debt - excluding current portion    
Maturity Date 2015 [1]  
Commercial Paper [Member]    
Short-term debt    
Weighted average interest rate 0.30%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [2] 0.40%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 [2]
Outstanding 1,175us-gaap_DebtCurrent
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember		 1,171us-gaap_DebtCurrent
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_CommercialPaperMember
Current Portion Long-Term Debt [Member]    
Short-term debt    
Maturity Date 2015  
Weighted average interest rate 5.00%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= gas_CurrentPortionLongTermDebtMember		 [2]    [2]
Outstanding 200us-gaap_DebtCurrent
/ us-gaap_LongtermDebtTypeAxis
= gas_CurrentPortionLongTermDebtMember		  
Long-term debt - excluding current portion    
Maturity Date 2015  
Senior Notes [Member] | Minimum [Member]    
Short-term debt    
Maturity Date 2016  
Long-term debt - excluding current portion    
Maturity Date 2016  
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%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 [2] 5.00%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 [2]
Outstanding 2,625us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 2,825us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
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%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= gas_FirstMortgageBondsMember		 [2] 5.60%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= gas_FirstMortgageBondsMember		 [2]
Outstanding 500us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_FirstMortgageBondsMember		 500us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_FirstMortgageBondsMember
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 0.90%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= gas_GasFacilityRevenueBondsMember		 [2] 1.00%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= gas_GasFacilityRevenueBondsMember		 [2]
Outstanding 200us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_GasFacilityRevenueBondsMember		 200us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_GasFacilityRevenueBondsMember
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%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_MediumTermNotesMember		 [2] 7.80%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_MediumTermNotesMember		 [2]
Outstanding 181us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_MediumTermNotesMember		 181us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_MediumTermNotesMember
Long-term Debt Principal [Member]    
Long-term debt - excluding current portion    
Weighted average interest rate 4.90%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= gas_LongTermDebtPrincipalMember		 [2] 4.90%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= gas_LongTermDebtPrincipalMember		 [2]
Outstanding 3,506us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_LongTermDebtPrincipalMember		 3,706us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_LongTermDebtPrincipalMember
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    [2],[3]    [2],[3]
Outstanding 80us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_LongTermDebtFairValueAdjustmentMember		 [3] 91us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_LongTermDebtFairValueAdjustmentMember		 [3]
Unamortized Debt Premium Discount, Net [Member]    
Short-term debt    
Maturity Date     
Long-term debt - excluding current portion    
Maturity Date     
Weighted average interest rate    [2]    [2]
Outstanding 16us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_UnamortizedDebtPremiumDiscountNetMember		 16us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_UnamortizedDebtPremiumDiscountNetMember
Long-term Debt Non-principal [Member]    
Long-term debt - excluding current portion    
Weighted average interest rate    [2]    [2]
Outstanding $ 96us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_LongTermDebtNonPrincipalMember		 $ 107us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= gas_LongTermDebtNonPrincipalMember
[1] As of December 31, 2014, the effective interest rates on our commercial paper borrowings were 0.5% for AGL Capital and 0.4% for Nicor Gas.
[2] Interest rates are calculated based on the daily weighted average balance outstanding for the 12 months ended December 31, 2014 and 2013.
[3] See Note 4 for additional information on our fair value measurements.