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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - SALES TAX ACCOUNTING (Details) - USD ($) $ in Millions | 3 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014
Accounting Policies [Abstract] |  |
Sales tax | $ 7.2 | $ 11.8

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- Definition Sales Tax + References No definition available. + Details
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Namespace Prefix: | njr_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration