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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Schedule of Cash and Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Mar. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]        
Cash and cash equivalents     $ 23,537 $ 12,810
Restricted cash included in other long-term assets $ 20,126 $ 20,081 $ 20,126 $ 20,081
Total cash, cash equivalents and restricted cash $ 43,663 $ 32,891