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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, DEMAND FEES (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2012
Demand fees | $ 214.0 | $ 215.1 | $ 216.5
NJRES [Member] |  |  |
Demand fees | 122.0 | 123.0 | 129.8
NJNG [Member] |  |  |
Demand fees | $ 92.0 | $ 92.1 | $ 86.7
Minimum [Member] |  |  |
Storage and pipeline capacity, contract term | 1 year |  |
Maximum [Member] |  |  |
Storage and pipeline capacity, contract term | 10 years |  |

X
- 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
Name: | njr_DemandFees
Namespace Prefix: | njr_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Storage and Pipeline Capacity, Term of Contract + References No definition available. + Details
Name: | njr_StorageandPipelineCapacityTermofContract
Namespace Prefix: | njr_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration