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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - New Accounting Pronouncements (Details) - Unsecured Debt - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Jul. 31, 2020
Mar. 31, 2017
Convertible Senior Notes Due 2025        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Debt instrument, unamortized discount $ 68,744 $ 80,530    
Stated interest rate (as a percent) 0.75%   0.75%  
Convertible Senior Notes Due 2022        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Debt instrument, unamortized discount $ 3,806 $ 9,263    
Stated interest rate (as a percent) 2.375%   2.375% 2.375%