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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Principal Amount    
Fair value:    
Marketable securities $ 715 $ 818
Estimated Fair Value | Recurring | Level 1    
Fair value:    
Marketable securities 654 712
Estimated Fair Value | Recurring | Level 1 | Other Noncurrent Assets    
Fair value:    
Marketable securities $ 700 $ 700