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Commitments and Contingencies Commitments and Contingencies (Details) - Inventories Under Indexed Contracts [Member] - Mid-Tex Division [Member] | Jun. 30, 2017 MMcf
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Long-term Purchase Commitment [Line Items] |
Long Term Purchase Commitment Minimum Quantity Required Within One Year | 53,200
Long Term Purchase Commitment Minimum Quantity Required One To Three Years | 37,600
Long Term Purchase Commitment Minimum Quantity Required After Three Years | 400

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- Definition Volume that must be purchased after three years following the balance sheet date to avoid penalties or unfavorable effects on pricing, among other adverse consequences. + References No definition available. + Details
Name: | ato_LongTermPurchaseCommitmentMinimumQuantityRequiredAfterThreeYears
Namespace Prefix: | ato_
Data Type: | num:volumeItemType
Balance Type: | na
Period Type: | instant

X
- Definition Volume that must be purchased within one to three years following the balance sheet date to avoid penalties or unfavorable effects on pricing, among other adverse consequences. + References No definition available. + Details
Name: | ato_LongTermPurchaseCommitmentMinimumQuantityRequiredOneToThreeYears
Namespace Prefix: | ato_
Data Type: | num:volumeItemType
Balance Type: | na
Period Type: | instant

X
- Definition Volume that must be purchased within one year of the balance sheet date to avoid penalties or unfavorable effects on pricing, among other adverse consequences. + References No definition available. + Details
Name: | ato_LongTermPurchaseCommitmentMinimumQuantityRequiredWithinOneYear
Namespace Prefix: | ato_
Data Type: | num:volumeItemType
Balance Type: | na
Period Type: | instant

X
- 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: | us-gaap_LongTermPurchaseCommitmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=ato_InventoriesUnderIndexedContractsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ato_MidTexDivisionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |