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Note 12 - Income Taxes (Details) - Income Tax Reconciliation (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Reconciliation [Abstract]      
Computed tax expense at statutory rate $ 325us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate $ 165us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate $ 151us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
State income tax, net of federal income tax benefit 36us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes 20us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes 19us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Tax effect of net income attributable to the noncontrolling interest (7)us-gaap_IncomeTaxReconciliationMinorityInterestIncomeExpense (7)us-gaap_IncomeTaxReconciliationMinorityInterestIncomeExpense (6)us-gaap_IncomeTaxReconciliationMinorityInterestIncomeExpense
Amortization of investment tax credits (2)us-gaap_IncomeTaxReconciliationTaxCreditsInvestment (3)us-gaap_IncomeTaxReconciliationTaxCreditsInvestment (3)us-gaap_IncomeTaxReconciliationTaxCreditsInvestment
Affordable housing credits (2)gas_AffordableHousingCredits (2)gas_AffordableHousingCredits (2)gas_AffordableHousingCredits
Flexible dividend deduction (2)us-gaap_IncomeTaxReconciliationDeductionsDividends (2)us-gaap_IncomeTaxReconciliationDeductionsDividends (2)us-gaap_IncomeTaxReconciliationDeductionsDividends
Sale of Compass Energy   6us-gaap_IncomeTaxReconciliationDispositionOfBusiness  
Other 2gas_ChangeInControlPayments    
Total income tax expense on Consolidated Statements of Income $ 350us-gaap_IncomeTaxExpenseBenefit $ 177us-gaap_IncomeTaxExpenseBenefit $ 157us-gaap_IncomeTaxExpenseBenefit