SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - CUSTOMER ACCOUNTS RECEIVABLE (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 325,406	$ 196,467
Receivable by subsidiary, percentage	100.00%	100.00%
Unbilled revenues	$ 70,138	$ 7,202
Energy Services
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 246,203	$ 150,322
Receivable by subsidiary, percentage	76.00%	77.00%
Natural Gas Distribution
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 72,067	$ 37,432
Receivable by subsidiary, percentage	22.00%	19.00%
Clean Energy Ventures
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 3,072	$ 2,655
Receivable by subsidiary, percentage	1.00%	1.00%
NJRHS and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Billed	$ 4,064	$ 6,058
Receivable by subsidiary, percentage	1.00%	3.00%