DEBT, SHORT-TERM BANK FACILITIES (Details) - USD ($)		12 Months Ended
		Sep. 30, 2016		Sep. 30, 2015
NJR | Letters of credit
Short-term Debt [Line Items]
Letters of credit outstanding, amount		$ 14,400,000		$ 16,500,000
NJR | Notes payable to banks
Short-term Debt [Line Items]
Outstanding at end of period		$ 121,700,000		$ 39,350,000
Weighted average interest rate at end of period		1.43%		1.17%
Amount available at end of period	[1]	$ 288,910,000		$ 369,176,000
NJR | Bank revolving credit facilities
Short-term Debt [Line Items]
Amount available at end of period		0		100,000,000
NJR | Committed Credit Facilities Due September 2020 | Bank revolving credit facilities
Short-term Debt [Line Items]
Bank revolving credit facilities	[2]	$ 425,000,000		$ 425,000,000
NJR | Bank revolving credit facilities
Short-term Debt [Line Items]
Commitment fee percentage		0.075%		0.075%
NJR | Uncommitted Credit Facilities | Bank revolving credit facilities
Short-term Debt [Line Items]
Bank revolving credit facilities	[3]	$ 0		$ 100,000,000
NJNG | Commercial paper
Short-term Debt [Line Items]
Outstanding at end of period		$ 0		$ 27,000,000
Weighted average interest rate at end of period		0.00%		0.20%
Amount available at end of period	[4]	$ 249,269,000		$ 222,269,000
NJNG | Letters of credit
Short-term Debt [Line Items]
Letters of credit outstanding, amount		$ 731,000		731,000
NJNG | Bank revolving credit facilities
Short-term Debt [Line Items]
Commitment fee percentage		0.075%
NJNG | Committed Credit Facilities Due May 2019 | Bank revolving credit facilities
Short-term Debt [Line Items]
Bank revolving credit facilities		$ 250,000,000	[2]	$ 250,000,000	[5]

[1] Letters of credit outstanding total $14.4 million and $16.5 million as of September 30, 2016 and 2015, respectively, which reduces amount available by the same amount.

[2] Committed credit facilities, which require commitment fees of .075 percent on the unused amounts.

[3] Uncommitted credit facilities, which require no commitment fees.

[4] Letters of credit outstanding total $731,000 as of September 30, 2016 and 2015, which reduces amount available by the same amount.

[5] Committed credit facilities, which require commitment fees of .075 percent on the unused amounts.