SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ASSETS HELD FOR SALE (Details) $ in Thousands		Jun. 01, 2018 MW	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Assets and Liabilities Held for Sale [Roll Forward]
Liabilities held for sale			$ 4,182	$ 0
Clean Energy Ventures
Long Lived Assets Held-for-sale [Line Items]
Wind farm total capacity (in MW) | MW		9.7
Assets and Liabilities Held for Sale [Roll Forward]
Property, plant and equipment - wind equipment, at cost			224,356	0
Property, plant and equipment - accumulated depreciation, wind equipment			(18,501)	0
Prepaid and accrued taxes			789	0
Other noncurrent assets			261	0
Asset held for sale			206,905	0
Accounts payable and other	[1]		186	0
Asset retirement obligation			3,996	0
Liabilities held for sale			$ 4,182	0
Assets reclassified as held for sale | Clean Energy Ventures
Assets and Liabilities Held for Sale [Roll Forward]
Property, plant and equipment - wind equipment, at cost				245,044
Property, plant and equipment - accumulated depreciation, wind equipment				(21,561)
Prepaid and accrued taxes				866
Other noncurrent assets				261
Asset held for sale				224,610
Accounts payable and other	[1]			0
Asset retirement obligation				4,262
Liabilities held for sale				4,262
Assets Sold | Clean Energy Ventures
Assets and Liabilities Held for Sale [Roll Forward]
Property, plant and equipment - wind equipment, at cost				(20,688)
Property, plant and equipment - accumulated depreciation, wind equipment				3,060
Prepaid and accrued taxes				(77)
Other noncurrent assets				0
Asset held for sale				(17,705)
Accounts payable and other	[1]			186
Asset retirement obligation				(266)
Liabilities held for sale				$ (80)

[1] Transaction fee owed to broker for the sale of Two Dot wind farm.