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Note 3 - Regulated Operations (Details) - Estimated Recognition of Rate Making Assets (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Regulatory Assets [Line Items]    
Regulatory Asset $ 714us-gaap_RegulatoryAssets $ 819us-gaap_RegulatoryAssets
Regulatory Asset Off Balance Sheet [Member] | Atlanta Gas Light [Member]    
Regulatory Assets [Line Items]    
Regulatory Asset 113us-gaap_RegulatoryAssets
/ dei_LegalEntityAxis
= gas_AtlantaGasLightMember
/ us-gaap_RegulatoryAssetAxis
= gas_RegulatoryAssetOffBalanceSheetMember		 80us-gaap_RegulatoryAssets
/ dei_LegalEntityAxis
= gas_AtlantaGasLightMember
/ us-gaap_RegulatoryAssetAxis
= gas_RegulatoryAssetOffBalanceSheetMember
Regulatory Asset Off Balance Sheet [Member] | Virginia Natural Gas [Member]    
Regulatory Assets [Line Items]    
Regulatory Asset 12us-gaap_RegulatoryAssets
/ dei_LegalEntityAxis
= gas_VirginiaNaturalGasMember
/ us-gaap_RegulatoryAssetAxis
= gas_RegulatoryAssetOffBalanceSheetMember		 12us-gaap_RegulatoryAssets
/ dei_LegalEntityAxis
= gas_VirginiaNaturalGasMember
/ us-gaap_RegulatoryAssetAxis
= gas_RegulatoryAssetOffBalanceSheetMember
Regulatory Asset Off Balance Sheet [Member] | Elizabethtown Gas [Member]    
Regulatory Assets [Line Items]    
Regulatory Asset 2us-gaap_RegulatoryAssets
/ dei_LegalEntityAxis
= gas_ElizabethtownGasMember
/ us-gaap_RegulatoryAssetAxis
= gas_RegulatoryAssetOffBalanceSheetMember		 1us-gaap_RegulatoryAssets
/ dei_LegalEntityAxis
= gas_ElizabethtownGasMember
/ us-gaap_RegulatoryAssetAxis
= gas_RegulatoryAssetOffBalanceSheetMember
Regulatory Asset Off Balance Sheet [Member]    
Regulatory Assets [Line Items]    
Regulatory Asset $ 127us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= gas_RegulatoryAssetOffBalanceSheetMember		 $ 93us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= gas_RegulatoryAssetOffBalanceSheetMember