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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, DEMAND FEES (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2014
Demand fees | $ 218.8 | $ 211.1 | $ 214.0
NJRES |  |  |
Demand fees | 141.0 | 130.6 | 122.0
NJNG |  |  |
Demand fees | $ 77.8 | $ 80.5 | $ 92.0
Minimum |  |  |
Storage and pipeline capacity, contract term | 1 year |  |
Maximum |  |  |
Storage and pipeline capacity, contract term | 10 years |  |

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- Definition Demand fees incurred associated with contracts for storage and/or pipeline capacity and recognized as a component of gas purchases. + References No definition available. + Details
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Period Type: | duration

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- Definition Storage and Pipeline Capacity, Term of Contract + 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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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=njr_NjngMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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