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

Maximum available borrowing capacity under the credit facility with consideration of any additional commited 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

Line Of Credit Facility, Number of Credit Facilities

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
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

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

- Definition

Public Utilities, Actual Debt Capital Structure, Percentage

+ References

No definition available.

+ Details

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Namespace Prefix:	ato_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(2))
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-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ 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:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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+ Details

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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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6036836-161870

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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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

No definition available.

+ Details

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

- 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

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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+ Details

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

- 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

+ 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

+ Details

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

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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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:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesApprovedEquityCapitalStructurePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_Unsecured3.375SeniorNotesdue2049Member
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_FiveYearUnsecuredRevolvingCreditAgreementMember
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Data Type:	na
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_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_A200MillionTermLoanAgreementMember
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Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
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Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_A10MillionRevolvingCreditNoteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_A50MillionRevolvingCreditFacilityMember
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_A600MillionRevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_A25MillionBankLoanAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_A50MillionBankLoanAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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- Details

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Debt - Narrative (Details)						12 Months Ended
Debt - Narrative (Details)	Oct. 01, 2020 USD ($)	Apr. 30, 2020 USD ($)	Apr. 23, 2020 USD ($)	Apr. 09, 2020 USD ($)	Oct. 02, 2019 USD ($)	Sep. 30, 2020 USD ($) credit_facilities	Apr. 01, 2020 USD ($)	Sep. 30, 2019 USD ($)
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 2,200,000,000
Ratio of total debt to total capitalization maximum						70.00%
Total-debt-to-total-capitalization ratio						42.00%
Unsecured 2.625% Senior Notes, due 2029
Debt Instrument [Line Items]
Interest rate						2.625%
Unsecured 3.375% Senior Notes, due 2049
Debt Instrument [Line Items]
Interest rate						3.375%
Five Year Unsecured Revolving Credit Agreement
Debt Instrument [Line Items]
Commercial paper						$ 0		$ 464,900,000
Weighted average interest rate								2.24%
Senior Notes
Debt Instrument [Line Items]
Proceeds from issuance of debt					$ 791,700,000
Senior Notes \| Unsecured 2.625% Senior Notes, due 2029
Debt Instrument [Line Items]
Debt face amount					$ 300,000,000
Interest rate					2.625%
Effective rate					2.72%
Senior Notes \| Unsecured 3.375% Senior Notes, due 2049
Debt Instrument [Line Items]
Debt face amount					$ 500,000,000
Interest rate					3.375%
Effective rate					3.42%
Term Loan Agreement \| $200 Million Term Loan Agreement
Debt Instrument [Line Items]
Debt face amount				$ 200,000,000
Minimum
Debt Instrument [Line Items]
Equity-to-capitalization ratio						50.00%
Outstanding indebtedness						$ 15,000,000
Maximum
Debt Instrument [Line Items]
Equity-to-capitalization ratio						60.00%
Outstanding indebtedness						$ 100,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Number of credit facilities \| credit_facilities						4
Revolving Credit Facility \| $10 Million Revolving Credit Note
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 10,000,000
Debt term						364 days
Revolving Credit Facility \| $50 Million Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 50,000,000				$ 50,000,000
Debt term		364 days
Remaining borrowing capacity						44,400,000
Revolving Credit Facility \| $600 Million Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 600,000,000
Debt term			364 days
Line of credit borrowings outstanding						0
Line of Credit \| $25 Million Bank Loan Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 25,000,000
Debt term						364 days
Line of Credit \| $50 Million Bank Loan Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity							$ 50,000,000
Line of credit borrowings outstanding						$ 0
Line of Credit \| $50 Million Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit borrowings outstanding						0
Commercial Paper \| Five Year Unsecured Revolving Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 1,500,000,000
Debt term						5 years
Accordion feature						$ 250,000,000
Loan increase potential						$ 1,750,000,000
London Interbank Offered Rate (LIBOR) \| Term Loan Agreement \| $200 Million Term Loan Agreement
Debt Instrument [Line Items]
Basis spread percent				125.00%
London Interbank Offered Rate (LIBOR) \| Revolving Credit Facility \| Minimum \| $600 Million Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread percent			0.00%
London Interbank Offered Rate (LIBOR) \| Revolving Credit Facility \| Maximum \| $600 Million Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Basis spread percent			125.00%
London Interbank Offered Rate (LIBOR) \| Commercial Paper \| Minimum \| Five Year Unsecured Revolving Credit Agreement
Debt Instrument [Line Items]
Basis spread percent						0.00%
London Interbank Offered Rate (LIBOR) \| Commercial Paper \| Maximum \| Five Year Unsecured Revolving Credit Agreement
Debt Instrument [Line Items]
Basis spread percent						1.25%
Subsequent Event \| Senior Notes
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 592,500,000
Subsequent Event \| Senior Notes \| Unsecured 1.50% Senior Notes, Due 2031
Debt Instrument [Line Items]
Debt face amount	$ 600,000,000
Interest rate	1.50%
Effective rate	1.71%