SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - GAS IN STORAGE (Details) $ in Thousands	Jun. 30, 2017 USD ($) Bcf	Sep. 30, 2016 USD ($) Bcf
Inventory [Line Items]
Gas in Storage, value | $	$ 201,682	$ 206,251
Gas in Storage, Bcf | Bcf	70.2	83.3
NJRES
Inventory [Line Items]
Gas in Storage, value | $	$ 150,303	$ 130,493
Gas in Storage, Bcf | Bcf	54.9	62.0
NJNG
Inventory [Line Items]
Gas in Storage, value | $	$ 51,379	$ 75,758
Gas in Storage, Bcf | Bcf	15.3	21.3