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Fair Value Measurements Available-For-Sale Securities (Details) (USD $)
Dec. 31, 2014
Sep. 30, 2014
Schedule Of Available For Sale Securities [Line Items]    
Available For Sale Securities Amortized Cost $ 69,401,000us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 67,466,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
Available For Sale Securities Gross Unrealized Gain Accumulated In Investments 10,528,000ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments 12,160,000ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
Gross Unrealized Loss (137,000)ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments (13,000)ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Fair value 79,792,000us-gaap_AvailableForSaleSecurities 79,613,000us-gaap_AvailableForSaleSecurities
Available-for-sale Securities, Other Disclosure Items [Abstract]    
Available-for-sale Securities, Supplemental Executive Benefit Plans 46,200,000ato_AvailableForSaleSecuritiesSupplementalExecutiveBenefitPlans 46,200,000ato_AvailableForSaleSecuritiesSupplementalExecutiveBenefitPlans
Equity Funds Domestic [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available For Sale Securities Amortized Cost 29,231,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsDomesticMember
26,633,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsDomesticMember
Available For Sale Securities Gross Unrealized Gain Accumulated In Investments 8,956,000ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsDomesticMember
10,136,000ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsDomesticMember
Gross Unrealized Loss (101,000)ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsDomesticMember
0ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsDomesticMember
Fair value 38,086,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsDomesticMember
36,769,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsDomesticMember
Equity Funds Foreign [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available For Sale Securities Amortized Cost 5,512,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsForeignMember
5,382,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsForeignMember
Available For Sale Securities Gross Unrealized Gain Accumulated In Investments 1,462,000ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsForeignMember
1,863,000ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsForeignMember
Gross Unrealized Loss 0ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsForeignMember
0ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsForeignMember
Fair value 6,974,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsForeignMember
7,245,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= ato_EquityFundsForeignMember
Debt Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available For Sale Securities Amortized Cost 33,474,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
33,266,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Available For Sale Securities Gross Unrealized Gain Accumulated In Investments 110,000ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
161,000ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Gross Unrealized Loss (36,000)ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
(13,000)ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value 33,548,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
33,414,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Money Market Funds [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available For Sale Securities Amortized Cost 1,184,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MoneyMarketFundsMember
2,185,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MoneyMarketFundsMember
Available For Sale Securities Gross Unrealized Gain Accumulated In Investments 0ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MoneyMarketFundsMember
0ato_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MoneyMarketFundsMember
Gross Unrealized Loss 0ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MoneyMarketFundsMember
0ato_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MoneyMarketFundsMember
Fair value $ 1,184,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MoneyMarketFundsMember
$ 2,185,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MoneyMarketFundsMember