SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - SALE-LEASEBACKS (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) asset	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Sale Leaseback Transaction [Line Items]
Proceeds from sale-leaseback transaction		$ 9,587	$ 7,107	$ 7,216
Proceeds from sale-leaseback transaction - solar		32,901	0	0
NJNG
Sale Leaseback Transaction [Line Items]
Proceeds from sale-leaseback transaction		$ 9,587	$ 7,100	$ 7,200
Minimum | NJNG
Sale Leaseback Transaction [Line Items]
Term of lease		7 years
Maximum | NJNG
Sale Leaseback Transaction [Line Items]
Term of lease		11 years
Clean Energy Ventures
Sale Leaseback Transaction [Line Items]
Term of lease	7 years
Proceeds from sale-leaseback transaction - solar	$ 32,901
Number of commercial solar assets sold | asset	2