Shareholders' Equity - Schedule of Forward Sales Agreement (Details) $ / shares in Units, $ in Thousands	6 Months Ended
Mar. 31, 2025 USD ($) $ / shares shares
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	12,598,259
Net proceeds available \| $	$ 1,659,625
Forward price (USD per share) \| $ / shares	$ 131.73
Forward Sales Equity Agreement Maturing Quarter Ended June 30, 2025
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	630,514
Net proceeds available \| $	$ 73,406
Forward price (USD per share) \| $ / shares	$ 116.42
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 \| $	$ 96,313
Forward price (USD per share) \| $ / shares	$ 118.08
Forward Sales Equity Agreement Maturing Quarter Ended December 31, 2025
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	2,344,567
Net proceeds available \| $	$ 297,650
Forward price (USD per share) \| $ / shares	$ 126.95
Forward Sales Equity Agreement Maturing Quarter Ended March 31, 2026
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	3,627,033
Net proceeds available \| $	$ 463,284
Forward price (USD per share) \| $ / shares	$ 127.73
Forward Sales Equity Agreement Maturing Quarter June 30, 2026
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	669,043
Net proceeds available \| $	$ 89,150
Forward price (USD per share) \| $ / shares	$ 133.25
Forward Sales Equity Agreement Maturing Quarter December 31, 2026
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	3,392,352
Net proceeds available \| $	$ 472,749
Forward price (USD per share) \| $ / shares	$ 139.36
Forward Sales Equity Agreement Maturing Quarter March 31, 2027
Forward Contract Indexed to Issuer's Equity [Line Items]
Shares available (in shares) \| shares	1,119,095
Net proceeds available \| $	$ 167,073
Forward price (USD per share) \| $ / shares	$ 149.29

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:

- Details

Name:	us-gaap_ForwardContractIndexedToIssuersEquityTypeAxis=ato_ForwardSalesEquityAgreementMaturingQuarterEndedMarch312026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ForwardContractIndexedToIssuersEquityTypeAxis=ato_ForwardSalesEquityAgreementMaturingQuarterJune302026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ForwardContractIndexedToIssuersEquityTypeAxis=ato_ForwardSalesEquityAgreementMaturingQuarterDecember312026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ForwardContractIndexedToIssuersEquityTypeAxis=ato_ForwardSalesEquityAgreementMaturingQuarterMarch312027Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: