SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ASSETS HELD FOR SALE (Details) - USD ($) $ in Thousands	Mar. 02, 2018	Mar. 31, 2018	Sep. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Property, plant and equipment - wind equipment, at cost		$ 3,091,779	$ 3,211,070
Prepaid and accrued taxes		14,395	24,764
Other noncurrent assets		69,549	74,818
Assets held for sale		224,898	$ 0
Clean Energy Ventures | 9.7 MW Wind Farm
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group consideration	$ 18,500
Gain on disposal, expected	$ 1,000
Clean Energy Ventures | Wind Assets And Related Liabilities
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Property, plant and equipment - wind equipment, at cost		244,972
Property, plant and equipment - accumulated depreciation, wind equipment		(21,561)
Prepaid and accrued taxes		1,226
Other noncurrent assets		261
Assets held for sale		224,900
Asset retirement obligation		$ 4,262