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CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (PARENTHETICALS) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Other Comprehensive Income Loss Tax Portion Attributable To Parent Abstract    
Unrealized holding gains (losses) on available-for-sale securities, tax $ (613)us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax $ 1,435us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax
Amortization and unrealized gains (losses) on interest rate agreements, tax (29,768)ato_OtherComprehensiveIncomeLossDerivativesQualifyingasHedgesInterestRateAgreementsTax 8,013ato_OtherComprehensiveIncomeLossDerivativesQualifyingasHedgesInterestRateAgreementsTax
Net unrealized gains (losses) on commodity cash flow hedging transactions, tax $ (18,696)ato_OtherComprehensiveIncomeLossDerivativesQualifyingasHedgesCommodityCashFlowHedgesTax $ 4,999ato_OtherComprehensiveIncomeLossDerivativesQualifyingasHedgesCommodityCashFlowHedgesTax