Shareholders' Equity - Schedule of Forward Sales Agreement (Details) $ / shares in Units, $ in Thousands	9 Months Ended
Jun. 30, 2024 USD ($) $ / shares shares
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	4,784,006
Net proceeds available \| $	$ 550,735
Forward price (USD per share) \| $ / shares	$ 115.12
Forward Sales Equity Agreement Maturing Quarter Ended June 30, 2025
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	3,931,418
Net proceeds available \| $	$ 450,884
Forward price (USD per share) \| $ / shares	$ 114.69
Forward Sales Equity Agreement Maturing Quarter Ended September 30, 2025
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	815,655
Net proceeds available \| $	$ 95,523
Forward price (USD per share) \| $ / shares	$ 117.11
Forward Sales Equity Agreement Maturing Quarter Ended December 31, 2025
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	36,933
Net proceeds available \| $	$ 4,328
Forward price (USD per share) \| $ / shares	$ 117.18

The per share price of the Company's stock at which the contract holder of the freestanding contract has the right to purchase or sell the Company's stock at a future date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 815
-SubTopic 40
-Section 50
-Paragraph 5
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (f)(1)
-SubTopic 10
-Topic 480
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481648/480-10-50-2

+ Details

Name:	us-gaap_ForwardContractIndexedToIssuersEquityForwardRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
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_ForwardContractIndexedToIssuersEquityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount that would be paid upon settlement of forward contract indexed to equity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (d)
-SubTopic 40
-Topic 815
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

+ Details

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

- Definition

Number of shares as a settlement alternative for each freestanding forward contract.

+ References

+ Details

Name:	us-gaap_ForwardContractIndexedToIssuersEquitySettlementAlternativesSharesAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_ForwardContractIndexedToIssuersEquityTypeAxis=ato_ForwardSalesEquityAgreementMaturingQuarterEndedJune302025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ForwardContractIndexedToIssuersEquityTypeAxis=ato_ForwardSalesEquityAgreementMaturingQuarterEndedSeptember302025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ForwardContractIndexedToIssuersEquityTypeAxis=ato_ForwardSalesEquityAgreementMaturingQuarterEndedDecember312025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: