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DERIVATIVE INSTRUMENTS - BROKER MARGIN DEPOSITS (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Sep. 30, 2016
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Natural Gas Distribution |  |
Broker margin - Current assets | $ 2,661 | $ 4,822
Energy Services |  |
Broker margin - Current assets | $ 23,166 | $ 42,822

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- Definition Amount of other receivables due from broker-dealers or clearing organizations. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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_NjresMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |