DERIVATIVE INSTRUMENTS - BALANCE SHEET RELATED DISCLOSURES (Details) - USD ($)	Mar. 31, 2017	Sep. 30, 2016	Jun. 01, 2015
Derivatives, Fair Value [Line Items]
Benchmark interest rate			3.26%
Notional amount of foreign currency derivatives			$ 125,000,000
Fair Value
Derivative assets, current	$ 32,103,000	$ 29,964,000
Derivative liabilities, current	32,559,000	61,080,000
Derivative assets, noncurrent	6,347,000	5,227,000
Derivative liabilities, noncurrent	6,105,000	25,252,000
Derivative assets	38,450,000	35,191,000
Derivative liabilities	38,664,000	86,332,000
Foreign currency contracts
Derivatives, Fair Value [Line Items]
Notional amount of foreign currency derivatives	2,200,000
Not Designated as Hedging Instrument
Fair Value
Derivative assets	38,450,000	35,191,000
Derivative liabilities	38,664,000	86,332,000
NJNG | Not Designated as Hedging Instrument | Physical commodity contracts
Fair Value
Derivative assets, current	64,000	235,000
Derivative liabilities, current	17,000	1,154,000
NJNG | Not Designated as Hedging Instrument | Financial commodity contracts
Fair Value
Derivative assets, current	2,240,000	805,000
Derivative liabilities, current	928,000	2,979,000
Derivative assets, noncurrent	0	75,000
Derivative liabilities, noncurrent	0	386,000
NJNG | Not Designated as Hedging Instrument | Interest rate contracts
Fair Value
Derivative assets, noncurrent	0	0
Derivative liabilities, noncurrent	4,392,000	23,073,000
NJRES | Not Designated as Hedging Instrument | Physical commodity contracts
Fair Value
Derivative assets, current	4,080,000	5,994,000
Derivative liabilities, current	2,460,000	11,660,000
Derivative assets, noncurrent	6,084,000	3,987,000
Derivative liabilities, noncurrent	588,000	1,212,000
NJRES | Not Designated as Hedging Instrument | Financial commodity contracts
Fair Value
Derivative assets, current	25,717,000	22,929,000
Derivative liabilities, current	29,127,000	45,255,000
Derivative assets, noncurrent	263,000	1,165,000
Derivative liabilities, noncurrent	1,125,000	581,000
NJRES | Not Designated as Hedging Instrument | Foreign currency contracts
Fair Value
Derivative assets, current	2,000	1,000
Derivative liabilities, current	27,000	$ 32,000
Series Ll | NJNG
Derivatives, Fair Value [Line Items]
Debt issued	$ 125,000,000
Interest rate, stated percentage	5.60%