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DEBT - NJNG SHORT-TERM DEBT (Details) - NJNG | Dec. 05, 2018 USD ($) | Sep. 30, 2020 USD ($) debt_instrument | Sep. 30, 2019 USD ($)
--- | --- | --- | ---
Letters of credit |  |  |
Line of Credit Facility [Line Items] |  |  |
Number of debt instruments (in debt instruments) | debt_instrument |  | 2 |
Letters of credit outstanding, amount |  | $ 731,000 | $ 731,000
Bank revolving credit facilities |  |  |
Line of Credit Facility [Line Items] |  |  |
Number of mutual options, extension period | 1 year |  |
Line of credit facility, maximum borrowing capacity, incremental increase | $ 50,000,000 |  |
Line of credit facility, maximum borrowing capacity, maximum increase | 100,000,000 |  |
Committed Credit Facilities Due May 2019 | Bank revolving credit facilities |  |  |
Line of Credit Facility [Line Items] |  |  |
Line of credit facility, maximum borrowing capacity | $ 250,000,000 |  |

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- Definition Line of Credit Facility, Maximum Borrowing Capacity, Incremental Increase + References No definition available. + Details
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Namespace Prefix: | njr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Line of Credit Facility, Maximum Borrowing Capacity, Maximum Increase + References No definition available. + Details
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Namespace Prefix: | njr_
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Balance Type: | credit
Period Type: | instant

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- Definition Number of Debt Instruments + References No definition available. + Details
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Namespace Prefix: | njr_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of Mutual Options, Extension Period + References No definition available. + Details
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Namespace Prefix: | njr_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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
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- 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 Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
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Period Type: | instant

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- Details
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Namespace Prefix: |
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- Details
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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