SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - SALE-LEASEBACKS (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Sale Leaseback Transaction [Line Items]
Proceeds from sale-leaseback transaction			$ 7,820	$ 9,587	$ 7,107
Proceeds from sale-leaseback transaction - solar			$ 71,538	32,901	0
Natural Gas Distribution
Sale Leaseback Transaction [Line Items]
Proceeds from sale-leaseback transaction	$ 7,800	$ 9,600			$ 7,100
Minimum | Natural Gas Distribution
Sale Leaseback Transaction [Line Items]
Term of lease			7 years
Maximum | Natural Gas Distribution
Sale Leaseback Transaction [Line Items]
Term of lease			11 years
Clean Energy Ventures
Sale Leaseback Transaction [Line Items]
Proceeds from sale-leaseback transaction - solar			$ 71,538	$ 32,901
Clean Energy Ventures | Minimum
Sale Leaseback Transaction [Line Items]
Term of lease			6 years
Clean Energy Ventures | Maximum
Sale Leaseback Transaction [Line Items]
Term of lease			15 years