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DEBT, NJNG SHORT-TERM DEBT (Details) - NJNG | 12 Months Ended |
--- | --- | ---
Sep. 30, 2016 USD ($) debt_instrument | Sep. 30, 2015 USD ($)
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 |
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 | [1] | $ 250,000,000 | [2]
Bank revolving credit facilities |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Debt instrument, term | 5 years |  |  |
Line of credit facility, maximum borrowing capacity, incremental increase | $ 15,000,000 |  |  |
Line of credit facility, maximum borrowing capacity, maximum increase | $ 50,000,000 |  |  |
[1] | Committed credit facilities, which require commitment fees of .075 percent on the unused amounts.
[2] | Committed credit facilities, which require commitment fees of .075 percent on the unused amounts.

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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_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentTerm
Namespace Prefix: | us-gaap_
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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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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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://www.xbrl.org/2003/role/presentationRef -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=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant

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- Details
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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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- Details
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Balance Type: |
Period Type: |