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Note 3 - Regulated Operations (Details) - Costs Related to Remediation of Former Operating Sites (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Environmental Exit Cost [Line Items]  
# of sites 46gas_NumberOfFormerOperatingSites
Probabilistic model cost estimates, low estimate $ 353us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesLowEstimate
Probabilistic model cost estimates, high estimate 724us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesHighEstimate
Engineering estimates 82us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesBestEstimate
Amount recorded 419us-gaap_EnvironmentalExitCostsCostsAccruedToDate [1]
Expected costs over next 12 months 93us-gaap_EnvironmentalExitCostsAnticipatedCost
Illinois Environmental [Member]  
Environmental Exit Cost [Line Items]  
# of sites 26gas_NumberOfFormerOperatingSites
/ us-gaap_StatementGeographicalAxis
= gas_IllinoisEnvironmentalMember		 [1],[2]
Probabilistic model cost estimates, low estimate 208us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesLowEstimate
/ us-gaap_StatementGeographicalAxis
= gas_IllinoisEnvironmentalMember
Probabilistic model cost estimates, high estimate 466us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesHighEstimate
/ us-gaap_StatementGeographicalAxis
= gas_IllinoisEnvironmentalMember
Engineering estimates 43us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesBestEstimate
/ us-gaap_StatementGeographicalAxis
= gas_IllinoisEnvironmentalMember		 [1]
Amount recorded 242us-gaap_EnvironmentalExitCostsCostsAccruedToDate
/ us-gaap_StatementGeographicalAxis
= gas_IllinoisEnvironmentalMember		 [1],[2]
Expected costs over next 12 months 45us-gaap_EnvironmentalExitCostsAnticipatedCost
/ us-gaap_StatementGeographicalAxis
= gas_IllinoisEnvironmentalMember		 [1],[2]
New Jersey Environmental [Member]  
Environmental Exit Cost [Line Items]  
# of sites 6gas_NumberOfFormerOperatingSites
/ us-gaap_StatementGeographicalAxis
= gas_NewJerseyEnvironmentalMember
Probabilistic model cost estimates, low estimate 105us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesLowEstimate
/ us-gaap_StatementGeographicalAxis
= gas_NewJerseyEnvironmentalMember
Probabilistic model cost estimates, high estimate 177us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesHighEstimate
/ us-gaap_StatementGeographicalAxis
= gas_NewJerseyEnvironmentalMember
Engineering estimates 14us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesBestEstimate
/ us-gaap_StatementGeographicalAxis
= gas_NewJerseyEnvironmentalMember
Amount recorded 112us-gaap_EnvironmentalExitCostsCostsAccruedToDate
/ us-gaap_StatementGeographicalAxis
= gas_NewJerseyEnvironmentalMember		 [1]
Expected costs over next 12 months 14us-gaap_EnvironmentalExitCostsAnticipatedCost
/ us-gaap_StatementGeographicalAxis
= gas_NewJerseyEnvironmentalMember
Cost recovery period 7 years
Georgia And Florida [Member]  
Environmental Exit Cost [Line Items]  
# of sites 13gas_NumberOfFormerOperatingSites
/ us-gaap_StatementGeographicalAxis
= gas_GeorgiaAndFloridaMember
Probabilistic model cost estimates, low estimate 40us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesLowEstimate
/ us-gaap_StatementGeographicalAxis
= gas_GeorgiaAndFloridaMember
Probabilistic model cost estimates, high estimate 81us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesHighEstimate
/ us-gaap_StatementGeographicalAxis
= gas_GeorgiaAndFloridaMember
Engineering estimates 15us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesBestEstimate
/ us-gaap_StatementGeographicalAxis
= gas_GeorgiaAndFloridaMember
Amount recorded 55us-gaap_EnvironmentalExitCostsCostsAccruedToDate
/ us-gaap_StatementGeographicalAxis
= gas_GeorgiaAndFloridaMember		 [1]
Expected costs over next 12 months 26us-gaap_EnvironmentalExitCostsAnticipatedCost
/ us-gaap_StatementGeographicalAxis
= gas_GeorgiaAndFloridaMember
Cost recovery period 5 years
North Carolina Environmental [Member]  
Environmental Exit Cost [Line Items]  
# of sites 1gas_NumberOfFormerOperatingSites
/ us-gaap_StatementGeographicalAxis
= gas_NorthCarolinaEnvironmentalMember		 [2]
Engineering estimates 10us-gaap_EnvironmentalExitCostsReasonablyPossibleAdditionalLossesBestEstimate
/ us-gaap_StatementGeographicalAxis
= gas_NorthCarolinaEnvironmentalMember		 [2]
Amount recorded 10us-gaap_EnvironmentalExitCostsCostsAccruedToDate
/ us-gaap_StatementGeographicalAxis
= gas_NorthCarolinaEnvironmentalMember		 [1],[2]
Expected costs over next 12 months $ 8us-gaap_EnvironmentalExitCostsAnticipatedCost
/ us-gaap_StatementGeographicalAxis
= gas_NorthCarolinaEnvironmentalMember		 [2]
Cost recovery period 0 years [2]
[1] Nicor Gas is responsible in whole or in part for 26 MGP sites, of which two sites have been remediated and their use is no longer restricted by the environmental condition of the property. Nicor Gas and Commonwealth Edison Company are parties to an agreement to cooperate in cleaning up residue at 23 of the sites. Nicor Gas' allocated share of cleanup costs for these sites is 52%.
[2] We have no regulatory recovery mechanism for the site in North Carolina. Therefore, there is no amount included within our regulatory assets and changes in estimated costs are recognized in income in the period of change.