Long Term Purchase Commitment, Per Unit

No definition available.

Name:	ato_LongTermPurchaseCommitmentPerUnit
Namespace Prefix:	ato_
Data Type:	dtr-types:monetaryPerVolumeItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Loss Contingency, Number of Fatalities Under Investigation

+ References

No definition available.

+ Details

Name:	ato_LossContingencyNumberOfFatalitiesUnderInvestigation
Namespace Prefix:	ato_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Loss Contingency, Number Of Incidents Investigated That Occurred During Period

+ References

No definition available.

+ Details

Name:	ato_LossContingencyNumberOfIncidentsInvestigatedThatOccurredDuringPeriod
Namespace Prefix:	ato_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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

- Definition

Volume that must be purchased within the specified time period of the agreement to avoid penalties or unfavorable effects on pricing, among other adverse consequences.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermPurchaseCommitmentMinimumVolumeRequired
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:volumeItemType
Balance Type:	na
Period Type:	duration

- Definition

Period covered by the long-term purchase commitment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_LongtermPurchaseCommitmentPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_SupplyCommitmentAxis=us-gaap_SupplyCommitmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ContractWithCustomerDurationAxis=us-gaap_ShortTermContractWithCustomerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ContractWithCustomerDurationAxis=ato_LongtermContractwithCustomerWithinTwoToThreeYearsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAgencyAxis=ato_NationalTransportationSafetyBoardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=ato_JacksonMississippiMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies (Details) $ in Millions		12 Months Ended
Commitments and Contingencies (Details) $ in Millions	Jan. 27, 2024 fatality incident	Sep. 30, 2025 USD ($) $ / Mcf MMcf	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Dec. 02, 2024 fatality
Long Term Purchase Commitment [Line Items]
Purchases under contract \| $		$ 153.4	$ 105.7	$ 182.0
Supply Commitment
Long Term Purchase Commitment [Line Items]
Contract term		1 year
Purchase commitment volume		21,000.0
Supply Commitment \| Weighted Average
Long Term Purchase Commitment [Line Items]
Fixed price contracts (USD per Mcf) \| $ / Mcf		2.70
Supply Commitment \| Short-term Contract with Customer
Long Term Purchase Commitment [Line Items]
Contract term		1 year
Purchase commitment volume		73,400
Supply Commitment \| Long-term Contract with Customer Within Two To Three Years
Long Term Purchase Commitment [Line Items]
Purchase commitment volume		114,900
Supply Commitment \| Long-term Contract with Customer Within Two To Three Years \| Minimum
Long Term Purchase Commitment [Line Items]
Contract term		2 years
Supply Commitment \| Long-term Contract with Customer Within Two To Three Years \| Maximum
Long Term Purchase Commitment [Line Items]
Contract term		3 years
National Transportation Safety Board
Long Term Purchase Commitment [Line Items]
Number of fatalities under investigation \| fatality	1				1
Jackson, Mississippi \| National Transportation Safety Board
Long Term Purchase Commitment [Line Items]
Number of incidents investigated that occurred during period \| incident	2