SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - CUSTOMER ACCOUNTS RECEIVABLE (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 298,824	$ 196,467
Receivable by subsidiary, percentage	100.00%	100.00%
Unbilled revenues	$ 55,405	$ 7,202
Energy Services
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 179,218	$ 150,322
Receivable by subsidiary, percentage	60.00%	77.00%
Natural Gas Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 112,884	$ 37,432
Receivable by subsidiary, percentage	38.00%	19.00%
Clean Energy Ventures
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 2,834	$ 2,655
Receivable by subsidiary, percentage	1.00%	1.00%
NJRHS and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 3,888	$ 6,058
Receivable by subsidiary, percentage	1.00%	3.00%