Name:	ato_ContractualDemandFeesDueFifthFiscalYear
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The amount as of the balance sheet date of contractual demand fees for contracted storage and transportation expected to be incurred during the fourth fiscal year following the balance sheet date.

No definition available.

Name:	ato_ContractualDemandFeesDueFourthFiscalYear
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The amount as of the balance sheet date of contractual demand fees for contracted storage and transportation expected to be incurred during the next fiscal year.

No definition available.

Name:	ato_ContractualDemandFeesDueNextYear
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The amount as of the balance sheet date of contractual demand fees for contracted storage and transportation expected to be incurred during the second fiscal year following the balance sheet date.

No definition available.

Name:	ato_ContractualDemandFeesDueSecondFiscalYear
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The amount as of the balance sheet date of contractual demand fees for contracted storage and transportation expected to be incurred during the years subsequent to the fifth fiscal year following the balacne sheet date.

No definition available.

Name:	ato_ContractualDemandFeesDueThereafter
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The amount as of the balance sheet date of contractual demand fees for contracted storage and transportation expected to be incurred during the third fiscal year following the balance sheet date.

No definition available.

Name:	ato_ContractualDemandFeesDueThirdFiscalYear
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Name:	ato_EstimatedContractualDemandFeesAbstract
Namespace Prefix:	ato_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.

Name:	ato_LongTermPurchaseCommitmentMinimumQuantityRequiredAfterThreeYears
Namespace Prefix:	ato_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

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.

No definition available.

Name:	ato_LongTermPurchaseCommitmentMinimumQuantityRequiredOneToThreeYears
Namespace Prefix:	ato_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

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.

No definition available.

Name:	ato_LongTermPurchaseCommitmentMinimumQuantityRequiredWithinOneYear
Namespace Prefix:	ato_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

The maximum per-unit amount the entity agreed to spend under fixed price purchase commitment arrangements.

No definition available.

Name:	ato_PurchaseCommitmentAmountMaximum
Namespace Prefix:	ato_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

The minimum per-unit amount the entity agreed to spend under fixed price purchase commitment arrangements.

No definition available.

Name:	ato_PurchaseCommitmentAmountMinimum
Namespace Prefix:	ato_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

Purchases made during the period under arrangements in which the entity has agreed to expend funds to procure goods or services from one or more suppliers.

No definition available.

Name:	ato_SignificantPurchaseCommitmentAmount
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The aggregate amount for the indivudual years as itemized in a table of estimated contractual demand fees for contracted storage and transportation as of the balance sheet date.

No definition available.

Name:	ato_TotalEstimatedContractualDemandFees
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=ato_InventoriesUnderFixedPriceContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

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