Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE MEASUREMENT - Financial Assets not Measured at Fair Value on a Recurring Basis (Details)

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FAIR VALUE MEASUREMENT - Financial Assets not Measured at Fair Value on a Recurring Basis (Details) - Nonrecurring - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Carrying Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total financial assets $ 4,976 $ 4,891
Carrying Value | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Notes receivable   4,891
Carrying Value | Level 3 | Notes receivable - current    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Notes receivable 32  
Carrying Value | Level 3 | Notes receivable - non-current    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Notes receivable 4,944  
Estimated Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total financial assets 4,976 4,891
Estimated Fair Value | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Notes receivable   $ 4,891
Estimated Fair Value | Level 3 | Notes receivable - current    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Notes receivable 32  
Estimated Fair Value | Level 3 | Notes receivable - non-current    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Notes receivable $ 4,944