SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Assets and Liabilities Held For Sale (Details) $ in Thousands	Jun. 30, 2020 USD ($) project	Dec. 31, 2019 USD ($) project
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of solar energy sites expected to be sold | project	2	3
Assets Held for Sale:
Current Assets	$ 40,044	$ 143,440
Liabilities Held for Sale:
Current Liabilities	5,862	6,043
Assets held for sale | Solar Assets, MTF, ACB, and ELK
Assets Held for Sale:
Current Assets	0	5,365
Net Utility Plant	19,335	18,692
Net Nonutility Property, Plant & Equipment	19,993	110,400
Goodwill	59	59
Regulatory Assets	638	415
Other Noncurrent Assets	19	8,509
Total Assets Held for Sale	40,044	143,440
Liabilities Held for Sale:
Current Liabilities	0	916
Asset Retirement Obligations	2,609	2,515
Regulatory Liabilities	3,246	2,583
Other Noncurrent Liabilities	7	29
Total Liabilities Held for Sale	$ 5,862	$ 6,043