AUGUST 2019 PUBLIC OFFERING AND PRIVATE PLACEMENT - Fair Value of Derivative Liability (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Change in fair value of warrant and private placement option liabilities	$ (12,131)	$ 4,850	$ (14,256)	$ 4,850
Level 3
Derivatives, Fair Value [Line Items]
Derivative liability, fair value at date of issuance	50,022		50,022		$ 64,278
Change in fair value of warrant and private placement option liabilities			(14,256)
Warrant Derivative Liability |  Level 3
Derivatives, Fair Value [Line Items]
Derivative liability, fair value at date of issuance	23,683		23,683		52,184
Change in fair value of warrant and private placement option liabilities			(28,501)
Private Placement Option Liability |  Level 3
Derivatives, Fair Value [Line Items]
Derivative liability, fair value at date of issuance	$ 26,339		26,339		$ 12,094
Change in fair value of warrant and private placement option liabilities			$ 14,245