DEBT - CREDIT FACILITIES (Details)			9 Months Ended
		Dec. 21, 2018 USD ($)	Jun. 30, 2019 USD ($) Mutual_option		Dec. 05, 2018 USD ($)	Sep. 30, 2018 USD ($)
Revolving Line of Credit, 4 Months
Line of Credit Facility [Line Items]
Bank revolving credit facilities		$ 100,000,000
Debt instrument, term		4 months
NJR | Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount			$ 4,200,000			$ 14,900,000
NJR | Notes
Line of Credit Facility [Line Items]
Amount outstanding at end of period			$ 0			$ 87,950,000
Weighted average interest rate at end of period			0.00%			3.07%
NJR | Revolving Credit Facility
Line of Credit Facility [Line Items]
Amount available at end of period	[1]		$ 420,775,000			$ 322,144,000
Debt, expiration, mutual options | Mutual_option			2
Debt, expiration, extension term			1 year
Line of credit facility, maximum borrowing capacity, incremental increase			$ 50,000,000
Line of credit facility, maximum borrowing capacity, maximum increase			250,000,000
NJR | Revolving Credit Facility | Committed Credit Facilities Due December 2023
Line of Credit Facility [Line Items]
Bank revolving credit facilities			425,000,000	[2]	$ 425,000,000	425,000,000	[2]
NJNG | Commercial Paper
Line of Credit Facility [Line Items]
Amount outstanding at end of period			$ 99,300,000			$ 64,000,000
Weighted average interest rate at end of period			2.62%			2.18%
Amount available at end of period	[3]		$ 149,969,000			$ 185,269,000
NJNG | Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount			$ 731,000			731,000
NJNG | Revolving Credit Facility
Line of Credit Facility [Line Items]
Debt, expiration, mutual options | Mutual_option			2
Debt, expiration, extension term			1 year
Line of credit facility, maximum borrowing capacity, incremental increase			$ 50,000,000
Line of credit facility, maximum borrowing capacity, maximum increase			100,000,000
NJNG | Revolving Credit Facility | Committed Credit Facilities Due December 2023
Line of Credit Facility [Line Items]
Bank revolving credit facilities					$ 250,000,000
NJNG | Revolving Credit Facility | Committed Credit Facilities Due May 2019
Line of Credit Facility [Line Items]
Bank revolving credit facilities	[2]		$ 250,000,000			$ 250,000,000

[1] Letters of credit outstanding total $4.2 million and $14.9 million for June 30, 2019 and September 30, 2018, respectively, which reduces amount available by the same amount.

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

[3] Letters of credit outstanding total $731,000 for both June 30, 2019 and September 30, 2018, which reduces the amount available by the same amount.