DEBT, SCHEDULE OF LONG-TERM DEBT (Details) - USD ($)	Sep. 30, 2016	Jun. 21, 2016	Mar. 22, 2016	Sep. 30, 2015	Apr. 15, 2015	Nov. 07, 2014	Mar. 13, 2014	Apr. 15, 2013
Debt Instrument [Line Items]
Total long-term debt	$ 1,063,550,000			$ 843,595,000
NJNG
Debt Instrument [Line Items]
Less: Current maturities of long-term debt	(11,452,000)			(11,138,000)
Total long-term debt	738,550,000			618,595,000
NJNG | First mortgage bonds: | Series II
Debt Instrument [Line Items]
Long-term debt	$ 10,300,000			10,300,000
Stated interest rate	4.50%
NJNG | First mortgage bonds: | Series JJ
Debt Instrument [Line Items]
Long-term debt	$ 10,500,000			10,500,000
Stated interest rate	4.60%
NJNG | First mortgage bonds: | Series KK
Debt Instrument [Line Items]
Long-term debt	$ 15,000,000			15,000,000
Stated interest rate	4.90%
NJNG | First mortgage bonds: | Series LL
Debt Instrument [Line Items]
Long-term debt	$ 125,000,000			125,000,000
Stated interest rate	5.60%
NJNG | First mortgage bonds: | Series MM
Debt Instrument [Line Items]
Long-term debt	$ 9,545,000			9,545,000
NJNG | First mortgage bonds: | Series NN
Debt Instrument [Line Items]
Long-term debt	41,000,000			41,000,000
NJNG | First mortgage bonds: | Series OO
Debt Instrument [Line Items]
Long-term debt	46,500,000			46,500,000
NJNG | First mortgage bonds: | Series PP
Debt Instrument [Line Items]
Long-term debt	$ 50,000,000			50,000,000
Stated interest rate	3.15%							3.15%
NJNG | First mortgage bonds: | Series QQ
Debt Instrument [Line Items]
Long-term debt	$ 70,000,000			70,000,000
Stated interest rate	3.58%						3.58%
NJNG | First mortgage bonds: | Series RR
Debt Instrument [Line Items]
Long-term debt	$ 55,000,000			55,000,000
Stated interest rate	4.61%						4.61%
NJNG | First mortgage bonds: | Series SS
Debt Instrument [Line Items]
Long-term debt	$ 50,000,000			50,000,000
Stated interest rate	2.82%				2.82%
NJNG | First mortgage bonds: | Series TT
Debt Instrument [Line Items]
Long-term debt	$ 100,000,000			100,000,000
Stated interest rate	3.66%				3.66%
NJNG | Series UU 3.63%
Debt Instrument [Line Items]
Long-term debt	$ 125,000,000			0
Stated interest rate	3.63%	3.63%
NJNG | Capital lease obligations | Capital lease obligation-buildings
Debt Instrument [Line Items]
Long-term debt	$ 14,262,000			16,700,000
NJNG | Capital lease obligations | Capital lease obligation-meters
Debt Instrument [Line Items]
Long-term debt	27,895,000			30,188,000
NJR
Debt Instrument [Line Items]
Less: Current maturities of long-term debt	(50,000,000)			0
Total long-term debt	325,000,000			225,000,000
NJR | Unsecured senior notes 6.05%
Debt Instrument [Line Items]
Long-term debt	$ 50,000,000			50,000,000
Stated interest rate	6.05%
NJR | Unsecured senior notes 2.51%
Debt Instrument [Line Items]
Long-term debt	$ 25,000,000			25,000,000
Stated interest rate	2.51%
NJR | Unsecured senior notes 3.25%
Debt Instrument [Line Items]
Long-term debt	$ 50,000,000			50,000,000
Stated interest rate	3.25%
NJR | Unsecured senior notes 3.48%
Debt Instrument [Line Items]
Long-term debt	$ 100,000,000			100,000,000		$ 100,000,000
Stated interest rate	3.48%					3.48%
NJR | Unsecured senior notes 3.20%
Debt Instrument [Line Items]
Long-term debt	$ 50,000,000			0
Stated interest rate	3.20%		3.20%
NJR | Unsecured senior notes 3.54%
Debt Instrument [Line Items]
Long-term debt	$ 100,000,000			$ 0
Stated interest rate	3.54%		3.54%