DEBT - NJR SHORT-TERM DEBT (Details)			12 Months Ended
		Dec. 14, 2017 USD ($)	Sep. 30, 2018 USD ($) debt_instrument Mutual_option	Apr. 18, 2018 USD ($)	Jan. 19, 2018 USD ($)	Sep. 30, 2017 USD ($)
Short-term Debt [Line Items]
Number of mutual options | Mutual_option			2
Number of mutual options, extension period			1 year
Letters of credit
Short-term Debt [Line Items]
Letters of credit outstanding, amount						$ 13,400,000
Revolving line of credit, 4 months
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity		$ 75,000,000
Line of credit facility, maximum borrowing capacity, incremental increase					$ 100,000,000
Debt instrument, term		4 months
Line of credit facility, fair value of amount outstanding				$ 0
NJR | Letters of credit
Short-term Debt [Line Items]
Number of debt instruments (in debt instruments) | debt_instrument			6
Letters of credit outstanding, amount			$ 14,900,000
NJR | 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			$ 11,900,000
NJR | 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
NJR | Committed Credit Facilities Due September 2020
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	[1]		425,000,000			$ 425,000,000
NJR | Bank revolving credit facilities
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity, incremental increase			5,000,000
Line of credit facility, maximum borrowing capacity, maximum increase			$ 100,000,000

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