SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - CUSTOMER ACCOUNTS RECEIVABLE (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 167,128	$ 142,658
Receivable by subsidiary, percentage	100.00%	100.00%
Unbilled revenues	$ 6,953	$ 5,744
NJRES
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 104,582	$ 102,884
Receivable by subsidiary, percentage	63.00%	72.00%
NJNG
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 56,302	$ 30,951
Receivable by subsidiary, percentage	34.00%	22.00%
NJRCEV
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 2,437	$ 1,807
Receivable by subsidiary, percentage	1.00%	1.00%
NJRHS and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 3,807	$ 7,016
Receivable by subsidiary, percentage	2.00%	5.00%