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Income Taxes - Summary of Changes in Valuation Allowances for Deferred Tax Assets (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022
Valuation Allowance [Roll Forward] |  |  |
Balance at Beginning of Year | $ 0 | $ 0 | $ 57,645
Additions | 0 | 0 | 0
Deductions | 0 | 0 | 57,645
Balance at End of Year | $ 0 | $ 0 | $ 0

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- Definition Amount of increase in the valuation allowance for a specified deferred tax asset. + References No definition available. + Details
Name: | nfg_ValuationAllowanceDeferredTaxAssetChangeInAmountAdditions
Namespace Prefix: | nfg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of decrease in the valuation allowance for a specified deferred tax asset. + References No definition available. + Details
Name: | nfg_ValuationAllowanceDeferredTaxAssetChangeInAmountDeductions
Namespace Prefix: | nfg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Valuation Allowance + References No definition available. + Details
Name: | nfg_ValuationAllowanceRollForward
Namespace Prefix: | nfg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482685/740-10-50-2 + Details
Name: | us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant