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Financial Instruments Commodity Contract Volume (Details) | Jun. 30, 2017 MMcf
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Nondesignated [Member] |
Commodity Contract Outstanding Volumes [Line Items] |
Investment Contract Volume | 18,833

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | ato_CommodityContractOutstandingVolumesLineItems
Namespace Prefix: | ato_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The volume of a commodity or material specified in an investment contract. + References No definition available. + Details
Name: | invest_InvestmentContractVolume
Namespace Prefix: | invest_
Data Type: | num:volumeItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |