SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Deferred Credits and Other Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Deferred credits and other liabilities
Transmission easements	$ 20,549	$ 20,966
Operating lease liabilities - noncurrent	1,846
Unearned revenue	4,217	4,592
Customer deposits	3,015	2,458
Financial liabilities - reclamation	12,091	4,938
Other	14,681	17,312
Total other deferred credits and other liabilities	56,399	$ 50,266
Transmission Right of Way Easements
Deferred credits and other liabilities
Total due for easement right of way	$ 31,200