DEBT - NJR SHORT-TERM DEBT (Details) - NJR	12 Months Ended
	Sep. 30, 2017 USD ($) debt_instrument	Sep. 30, 2016 USD ($)
Letters of credit
Short-term Debt [Line Items]
Number of debt instruments (in debt instruments) | debt_instrument	6
Letters of credit outstanding, amount	$ 13,400,000	$ 14,400,000
Letters of Credit on Behalf of NJRES
Short-term Debt [Line Items]
Number of debt instruments (in debt instruments) | debt_instrument	3
Letters of credit outstanding, amount	$ 10,400,000
Letters of Credit on Behalf of NJRCEV
Short-term Debt [Line Items]
Number of debt instruments (in debt instruments) | debt_instrument	3
Letters of credit outstanding, amount	$ 3,000,000
Committed Credit Facilities Due September 2020 | Bank revolving credit facilities
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 425,000,000	$ 425,000,000
Bank revolving credit facilities
Short-term Debt [Line Items]
Commitment fee percentage	0.075%	0.075%
Line of credit facility, maximum borrowing capacity, incremental increase	$ 5,000,000
Line of credit facility, maximum borrowing capacity, maximum increase	$ 100,000,000