DERIVATIVE INSTRUMENTS - BALANCE SHEET RELATED DISCLOSURES (Details) - USD ($)	Dec. 31, 2016	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	$ 50,215,000	$ 29,964,000
Derivative liabilities, current	93,080,000	61,080,000
Derivative assets, noncurrent	3,660,000	5,227,000
Derivative liabilities, noncurrent	4,527,000	25,252,000
Derivative assets	53,875,000	35,191,000
Derivative liabilities	97,607,000	86,332,000
Foreign currency contracts
Derivatives, Fair Value [Line Items]
Notional amount of foreign currency derivatives	3,700,000
Not Designated as Hedging Instrument
Fair Value
Derivative assets	53,875,000	35,191,000
Derivative liabilities	97,607,000	86,332,000
NJNG | Not Designated as Hedging Instrument | Physical commodity contracts
Fair Value
Derivative assets, current	1,181,000	235,000
Derivative liabilities, current	76,000	1,154,000
NJNG | Not Designated as Hedging Instrument | Financial commodity contracts
Fair Value
Derivative assets, current	13,931,000	805,000
Derivative liabilities, current	2,919,000	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	2,702,000	23,073,000
NJRES | Not Designated as Hedging Instrument | Physical commodity contracts
Fair Value
Derivative assets, current	5,477,000	5,994,000
Derivative liabilities, current	5,978,000	11,660,000
Derivative assets, noncurrent	2,192,000	3,987,000
Derivative liabilities, noncurrent	1,193,000	1,212,000
NJRES | Not Designated as Hedging Instrument | Financial commodity contracts
Fair Value
Derivative assets, current	29,626,000	22,929,000
Derivative liabilities, current	84,016,000	45,255,000
Derivative assets, noncurrent	1,468,000	1,165,000
Derivative liabilities, noncurrent	632,000	581,000
NJRES | Not Designated as Hedging Instrument | Foreign currency contracts
Fair Value
Derivative assets, current	0	1,000
Derivative liabilities, current	91,000	$ 32,000
First Mortgage | Series Ll | NJNG
Derivatives, Fair Value [Line Items]
Debt issued	$ 125,000,000
Interest rate, stated percentage	5.60%