Debt Instrument, Covenant Compliance, Default Provision Trigger Amount

No definition available.

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

- Definition

Monetary value of the accordion feature, the additional line of credit portion that could be borrowed against with the approval of the issuing banks.

+ References

No definition available.

+ Details

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

- Definition

Maximum available borrowing capacity under the credit facility with consideration of any additional committed line portions under the accordion feature that could be borrowed against with the approval of the issuing banks.

+ References

No definition available.

+ Details

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

- Definition

Number Of Credit Facilities

+ References

No definition available.

+ Details

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

- Definition

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

+ References

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-Section S99
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+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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+ Details

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

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 835
-SubTopic 30
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+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
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Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, 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_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
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+ Details

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

- 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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
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+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
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+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
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+ Details

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

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(i)(1))
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+ Details

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

- Definition

Percentage of public utility's approved debt to capital, including debt and equity.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Indebtedness divided by net capital.

+ References

No definition available.

+ Details

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Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_RangeAxis=srt_MinimumMember
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Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
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Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_Unsecured5.45SeniorNotesDueJanuary2056Member
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Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_FiveYearUnsecuredRevolvingCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_CommercialPaperMember
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Data Type:	na
Balance Type:
Period Type:

- Details

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Balance Type:
Period Type:

- Details

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Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_SecuredOvernightFinancingRateSofrMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_A1.5BillionRevolvingCreditFacilityMember
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Period Type:

- Details

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Balance Type:
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- Details

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Period Type:

- Details

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- Details

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Debt - Narrative (Details)			6 Months Ended
Debt - Narrative (Details)	Apr. 01, 2026 USD ($)	Oct. 01, 2025 USD ($)	Mar. 31, 2026 USD ($) credit_facility	Mar. 31, 2025 USD ($)	Sep. 30, 2025 USD ($)
Line Of Credit Facility [Line Items]
Proceeds from issuance of long-term debt			$ 596,532,000	$ 645,372,000
Maximum debt-to-total-capitalization ratio (in percentage)			70.00%
Debt-to-total-capitalization ratio (in percentage)			0.40
Minimum
Line Of Credit Facility [Line Items]
Outstanding indebtedness			$ 15,000,000
Maximum
Line Of Credit Facility [Line Items]
Outstanding indebtedness			$ 100,000,000
Unsecured 5.45% Senior Notes, due January 2056
Line Of Credit Facility [Line Items]
Interest rate (in percentage)			5.45%
Unsecured 5.45% Senior Notes, due January 2056 \| Senior Notes
Line Of Credit Facility [Line Items]
Debt face amount		$ 600,000,000
Interest rate (in percentage)		5.45%
Effective rate (in percentage)		4.85%
Proceeds from issuance of long-term debt		$ 590,000,000.0
Five Year Unsecured Revolving Credit Agreement \| Commercial Paper
Line Of Credit Facility [Line Items]
Maximum borrowing capacity			$ 1,500,000,000
Debt agreement term (in years)			5 years
Five Year Unsecured Revolving Credit Agreement, March 28, 2030
Line Of Credit Facility [Line Items]
Outstanding commercial paper			$ 0		$ 0
Five Year Unsecured Revolving Credit Agreement, March 28, 2030 \| Minimum \| Base Rate
Line Of Credit Facility [Line Items]
Interest rate spread			0.00%
Five Year Unsecured Revolving Credit Agreement, March 28, 2030 \| Minimum \| Term SOFR
Line Of Credit Facility [Line Items]
Interest rate spread			0.75%
Five Year Unsecured Revolving Credit Agreement, March 28, 2030 \| Maximum \| Base Rate
Line Of Credit Facility [Line Items]
Interest rate spread			0.25%
Five Year Unsecured Revolving Credit Agreement, March 28, 2030 \| Maximum \| Term SOFR
Line Of Credit Facility [Line Items]
Interest rate spread			1.25%
Five Year Unsecured Revolving Credit Agreement, March 28, 2030 \| Commercial Paper
Line Of Credit Facility [Line Items]
Maximum borrowing capacity			$ 1,500,000,000
Accordion feature			250,000,000
Maximum borrowing capacity post accordion feature			1,750,000,000
Five Year Unsecured Revolving Credit Agreement, March 28, 2030 \| Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity			$ 3,100,000,000
Number of credit facilities \| credit_facility			4
$1.5 Billion Revolving Credit Facility \| Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity			$ 1,500,000,000
Debt agreement term (in years)			3 years
Accordion feature			$ 250,000,000
Maximum borrowing capacity post accordion feature			1,750,000,000
Outstanding borrowings			$ 0		0
$1.5 Billion Revolving Credit Facility \| Revolving Credit Facility \| Minimum \| Base Rate
Line Of Credit Facility [Line Items]
Interest rate spread			0.00%
$1.5 Billion Revolving Credit Facility \| Revolving Credit Facility \| Minimum \| Term SOFR
Line Of Credit Facility [Line Items]
Interest rate spread			0.75%
$1.5 Billion Revolving Credit Facility \| Revolving Credit Facility \| Maximum \| Base Rate
Line Of Credit Facility [Line Items]
Interest rate spread			0.25%
$1.5 Billion Revolving Credit Facility \| Revolving Credit Facility \| Maximum \| Term SOFR
Line Of Credit Facility [Line Items]
Interest rate spread			1.25%
$50 Million Bank Loan Agreement \| Line of Credit
Line Of Credit Facility [Line Items]
Outstanding borrowings			$ 0		$ 0
$50 Million Bank Loan Agreement \| Line of Credit \| Subsequent Event
Line Of Credit Facility [Line Items]
Maximum borrowing capacity	$ 50,000,000
Debt agreement term (in years)	364 days
$50 Million Revolving Credit Facility
Line Of Credit Facility [Line Items]
Outstanding borrowings			0
$50 Million Revolving Credit Facility \| Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity			$ 50,000,000
Debt agreement term (in years)			364 days
Remaining borrowing capacity			$ 44,400,000