DEBT - SHORT-TERM BANK FACILITIES (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017
Letters of credit
Short-term Debt [Line Items]
Letters of credit outstanding, amount			$ 13,400
NJR | Letters of credit
Short-term Debt [Line Items]
Letters of credit outstanding, amount		$ 14,900
NJR | Notes payable to banks
Short-term Debt [Line Items]
Outstanding at end of period		$ 87,950	$ 255,000
Weighted average interest rate at end of period		3.07%	2.14%
Amount available at end of period	[1]	$ 322,144	$ 156,601
NJR | Committed Credit Facilities Due September 2020
Short-term Debt [Line Items]
Bank revolving credit facilities	[2]	$ 425,000	425,000
NJR | Bank revolving credit facilities
Short-term Debt [Line Items]
Commitment fee percentage		0.075%
Natural Gas Distribution | Commercial paper
Short-term Debt [Line Items]
Outstanding at end of period		$ 64,000	$ 11,000
Weighted average interest rate at end of period		2.18%	1.13%
Amount available at end of period	[3]	$ 185,269	$ 238,269
Natural Gas Distribution | Letters of credit
Short-term Debt [Line Items]
Letters of credit outstanding, amount		$ 731	$ 731
Natural Gas Distribution | Bank revolving credit facilities
Short-term Debt [Line Items]
Commitment fee percentage		0.075%

[1] Letters of credit outstanding total $14.9 million and $13.4 million as of September 30, 2018 and 2017, 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] Letters of credit outstanding total $731,000 as of September 30, 2018 and 2017, which reduces amount available by the same amount.