DEBT - Interest Expense (Details) - Convertible debt - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 29, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Debt Instrument [Line Items]
Interest rate	0.75%	0.75%	0.75%	0.75%	0.75%
Contractual interest expense (0.75% coupon)		$ 1,125	$ 1,125	$ 3,375	$ 2,700
Amortization of debt discount		0	3,807	0	9,060
Amortization of issuance costs		677	565	2,025	1,345
Interest expense		$ 1,802	$ 5,497	$ 5,400	$ 13,105
Weighted-average effective interest rate		1.20%	4.30%	1.20%	4.30%