XML 72 R51.htm IDEA: XBRL DOCUMENT
v3.25.3

Regulation (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Sep. 30, 2024
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 608,729 | $ 579,390
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 1,269,648 | 1,227,882
Regulatory excess deferred taxes |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 190,274 | 257,001
Regulatory cost of removal obligation |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 641,019 | 607,032
Deferred gas costs |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 6,879 | 9,142
APT annual adjustment mechanism |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 99,393 | 73,119
Pension and postretirement benefit costs |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 291,351 | 247,250
Other |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 40,732 | 34,338
Pension and postretirement benefit costs |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 262 | 11,243
Infrastructure mechanisms |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 314,047 | 246,734
Winter Storm Uri incremental costs |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 5,841 | 10,373
Deferred gas costs |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 140,626 | 159,762
Regulatory excess deferred taxes |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 49,793 | 51,380
Recoverable loss on reacquired debt |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 2,903 | 3,070
Deferred pipeline record collection costs |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 39,035 | 41,742
System Safety and Integrity Riders |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 43,625 | 38,632
Other |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 12,597 | $ 16,454

X
- 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_RegulatoryAssetLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 340 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478742/980-340-50-1 + Details
Name: | us-gaap_RegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 405 -Name Accounting Standards Codification -Section 25 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477571/980-405-25-1 + Details
Name: | us-gaap_RegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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_RegulatoryLiabilitiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_DeferredIncomeTaxChargesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_RemovalCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_DeferredFuelCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=ato_RegulatoryClauseRevenuesOverRecoveredMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_PensionAndOtherPostretirementPlansCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_OtherRegulatoryAssetsLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_PensionAndOtherPostretirementPlansCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=ato_InfrastructureMechanismsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_StormCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_DeferredFuelCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_DeferredIncomeTaxChargesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_LossOnReacquiredDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_DeferredProjectCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=ato_SystemSafetyAndIntegrityRidersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_OtherRegulatoryAssetsLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |