SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ASSETS HELD FOR SALE (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Sep. 30, 2018
Assets Held For Sale Activity [Roll Forward]
Liabilities held for sale:		$ 0	$ 4,182
Clean Energy Ventures
Assets Held For Sale Activity [Roll Forward]
Nonutility plant and equipment - wind equipment, at cost		0	224,356
Nonutility plant and equipment - accumulated depreciation, wind equipment		0	(18,501)
Prepaid and other current assets		0	789
Other noncurrent assets		0	261
Assets held for sale:		0	206,905
Accounts payable and other		0	186
Asset retirement obligation		0	3,996
Liabilities held for sale:		0	$ 4,182
Held-for-sale | Clean Energy Ventures
Assets Held For Sale Activity [Roll Forward]
Nonutility plant and equipment - wind equipment, at cost		0
Nonutility plant and equipment - accumulated depreciation, wind equipment		0
Prepaid and other current assets		1,747
Other noncurrent assets		0
Assets held for sale:		1,747
Accounts payable and other		0
Asset retirement obligation		0
Liabilities held for sale:		0
Assets sold | Clean Energy Ventures
Assets Held For Sale Activity [Roll Forward]
Nonutility plant and equipment - wind equipment, at cost		(224,356)
Nonutility plant and equipment - accumulated depreciation, wind equipment		18,501
Prepaid and other current assets		(1,541)
Other noncurrent assets		(261)
Assets held for sale:		(207,657)
Accounts payable and other		(186)
Asset retirement obligation		(3,996)
Liabilities held for sale:		(4,182)
Other adjustments | Clean Energy Ventures
Assets Held For Sale Activity [Roll Forward]
Nonutility plant and equipment - wind equipment, at cost	[1]	0
Nonutility plant and equipment - accumulated depreciation, wind equipment	[1]	0
Prepaid and other current assets	[1]	(995)
Other noncurrent assets	[1]	0
Assets held for sale:	[1]	(995)
Accounts payable and other	[1]	0
Asset retirement obligation	[1]	0
Liabilities held for sale:	[1]	$ 0

[1] Activity relates to amortization of prepaid and other current assets.