SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash, Cash Equivalents and Restricted Cash and Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash, cash equivalents and restricted cash
Cash and cash equivalents	$ 83,070	$ 116,858
Restricted cash and investments - current	182	126
Restricted Cash and Investments, Noncurrent	30,516	10,606
Cash, cash equivalents and restricted cash and investments	113,768	127,590	$ 150,965	$ 167,890
Internally restricted cash for the funding of deferred revenues and incomes held as regulatory liabilities	$ 25,500	$ 4,600