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DEBT - NJR SHORT-TERM DEBT (Details) - NJR | 12 Months Ended
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Sep. 30, 2017 USD ($) debt_instrument | Sep. 30, 2016 USD ($)
Letters of credit |  |
Short-term Debt [Line Items] |  |
Number of debt instruments (in debt instruments) | debt_instrument | 6 |
Letters of credit outstanding, amount | $ 13,400,000 | $ 14,400,000
Letters of Credit on Behalf of NJRES |  |
Short-term Debt [Line Items] |  |
Number of debt instruments (in debt instruments) | debt_instrument | 3 |
Letters of credit outstanding, amount | $ 10,400,000 |
Letters of Credit on Behalf of NJRCEV |  |
Short-term Debt [Line Items] |  |
Number of debt instruments (in debt instruments) | debt_instrument | 3 |
Letters of credit outstanding, amount | $ 3,000,000 |
Committed Credit Facilities Due September 2020 | Bank revolving credit facilities |  |
Short-term Debt [Line Items] |  |
Line of credit facility, maximum borrowing capacity | $ 425,000,000 | $ 425,000,000
Bank revolving credit facilities |  |
Short-term Debt [Line Items] |  |
Commitment fee percentage | 0.075% | 0.075%
Line of credit facility, maximum borrowing capacity, incremental increase | $ 5,000,000 |
Line of credit facility, maximum borrowing capacity, maximum increase | $ 100,000,000 |

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- Definition Line of Credit Facility, Maximum Borrowing Capacity, Incremental Increase + References No definition available. + Details
Name: | njr_LineOfCreditFacilityMaximumBorrowingCapacityIncrementalIncrease
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
Name: | njr_LineOfCreditFacilityMaximumBorrowingCapacityMaximumIncrease
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
Name: | njr_NumberofDebtInstruments
Namespace Prefix: | njr_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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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
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
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_
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
Name: | us-gaap_ShortTermDebtLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
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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: |
Data Type: | na
Balance Type: |
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- Details
Name: | us-gaap_DebtInstrumentAxis=njr_CommittedCreditFacilitiesDueSeptember2020Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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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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