SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - CUSTOMER ACCOUNTS RECEIVABLE (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 242,698	$ 142,658
Receivable by subsidiary, percentage	100.00%	100.00%
Unbilled revenues	$ 42,657	$ 5,744
NJRES
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 135,199	$ 102,884
Receivable by subsidiary, percentage	56.00%	72.00%
NJNG
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 100,449	$ 30,951
Receivable by subsidiary, percentage	41.00%	22.00%
NJRCEV
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 2,405	$ 1,807
Receivable by subsidiary, percentage	1.00%	1.00%
NJRHS and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 4,645	$ 7,016
Receivable by subsidiary, percentage	2.00%	5.00%