Name:	ato_DiscontinuedOperationsGrossProfit
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount, if any, of interest and miscellaneous expenses attirbuted to the discontinued operations during the reporting period.

No definition available.

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

The total amount of operating revenues recognized during the period for discontinued operations.

No definition available.

Name:	ato_DiscontinuedOperationsOperatingRevenues
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Cost of gas purchased during the reporting period for discontinued operations.

No definition available.

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

For the disposal group, including a component of the entity (discontinued operation), carrying amount as of the balance sheet date of natural gas stored underground in depleted gas reservoirs, aquifers, or salt caverns to meet seasonal and peak load demands, and also as insurance against unforeseen supply disruptions, and deemed to be a current asset because it is expected to be used within twelve months or in the normal operating cycle.

No definition available.

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

For the disposal group, including a component of the entity (discontinued operation), reflects the total obligation as of the balance sheet date to remove regulatory assets from service.

No definition available.

Name:	ato_DisposalGroupIncludingDiscontinuedOperationRegulatoryCostOfRemovalObligation
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The aggregate value for liabilities related to a disposal group that is held for sale and anticipated to be sold in less than one year. The liabilities are expected to be discharged as part of the plan of sale for the asset.

No definition available.

Name:	ato_LiabilitiesOfDisposalGroup
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The aggregate value (measured at the lower of net carrying value or fair value less cost of disposal) for assets of a disposal group, including a component of the entity (discontinued operation), to be sold or that has been disposed of through sale, as of the financial statement date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 46

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

Pretax income (loss) from operations of a business component (exclusive of any gain or loss on disposal, or provision therefore) during the reporting period, until its disposal.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 43

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 47

-Subparagraph c

Name:	us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

After tax income (loss) from operations of a business component (exclusive of any gain or loss on disposal, or provision therefore) during the reporting period, until its disposal.

Name:	us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Income tax expense (benefit) attributable to income (loss) from discontinued operations during the phase-out period.

Name:	us-gaap_DiscontinuedOperationTaxEffectOfIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

For the disposal group, including a component of the entity (discontinued operation), carrying value of obligations incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 47

-Subparagraph a

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationAccountsPayable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

Amount of operating expenses attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

No definition available.

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of operating income or loss attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

No definition available.

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

For the disposal group, including a component of the entity (discontinued operation), carrying amount of assets not otherwise specified in the taxonomy that are expected to be realized or consumed within one year or the normal operating cycle, if longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 47

-Subparagraph a

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

For the disposal group, including a component of the entity (discontinued operation), obligations not otherwise itemized that are due within one year or operating cycle, if longer, from the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 47

-Subparagraph a

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherCurrentLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

For the disposal group, including a component of the entity (discontinued operation), carrying amount as of the balance sheet date of assets not otherwise specified, and which are expected to be realized or consumed after one year or beyond the normal operating cycle, if longer. For an unclassified balance sheet, represents both current and noncurrent other assets.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 47

-Subparagraph a

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

For the disposal group, including a component of the entity (discontinued operation), obligations not otherwise itemized that are due after one year or operating cycle, if longer, from the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 47

-Subparagraph a

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherNoncurrentLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

For the disposal group, including a component of the entity (discontinued operation), carrying value (net of accumulated depreciation and any write-downs) of tangible assets that are held by an entity for use in the production or supply of goods and services, for rental to others, or for administrative purposes and that are expected to provide economic benefit for more than one year. Examples include land, buildings, vehicles and production equipment.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 47

-Subparagraph a