ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 91,028	$ 43,695
Restricted cash, current	2,788	0
Restricted cash, noncurrent	0	4,973
Total cash, cash equivalents and restricted cash shown in the statements of cash flows	93,816	$ 48,668	$ 45,029
Grant costs
Cash and Cash Equivalents [Line Items]
Restricted cash, noncurrent	1,700		$ 4,200
Release of restricted cash	(2,500)
Leasehold improvement costs
Cash and Cash Equivalents [Line Items]
Restricted cash, noncurrent	$ 1,100