SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - New Accounting Pronouncements (Details) - Unsecured Debt - USD ($) $ in Thousands	Jun. 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	$ 72,734	$ 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	$ 5,658	$ 9,263
Stated interest rate (as a percent)	2.375%		2.375%	2.375%