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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - GAS IN STORAGE (Details)
$ in Thousands
Dec. 31, 2016
USD ($)
Bcf
Sep. 30, 2016
USD ($)
Bcf
Inventory [Line Items]    
Gas in Storage, value | $ $ 220,097 $ 206,251
Gas in Storage, Bcf | Bcf 78.8 83.3
NJRES    
Inventory [Line Items]    
Gas in Storage, value | $ $ 163,393 $ 130,493
Gas in Storage, Bcf | Bcf 62.9 62.0
NJNG    
Inventory [Line Items]    
Gas in Storage, value | $ $ 56,704 $ 75,758
Gas in Storage, Bcf | Bcf 15.9 21.3