STOCKHOLDERS' EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		2 Months Ended	3 Months Ended		9 Months Ended
	Nov. 05, 2020	May 31, 2020	Nov. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Sep. 05, 2019
Preferred stock, shares authorized				10,000,000		10,000,000
Preferred stock, par value (in dollars per share)				$ 0.001		$ 0.001
Common Stock, Shares Authorized				150,000,000		150,000,000			150,000,000
Common stock, par value (in dollars per share)				$ 0.001		$ 0.001			$ 0.001
Offering costs				$ 10,900		$ 10,900		$ 10,900
Expenses on stock options				$ 28,330		46,768	$ 5,714
Warrant [Member]
Expenses on stock options						$ 0	0
Unvested Restricted stock [Member]
Restricted stock outstanding				9,909,709		9,909,709			7,091,789
Expenses on stock options				$ 27,500	$ 1,300	$ 41,600	3,400
Number of shares granted						3,897,829
Total unrecognized compensation expense related to unvested time-based restricted stock				55,000		$ 55,000
Weighted-average period of recognition for total unrecognized compensation expense related to unvested time-based restricted stock						1 year
Employee Stock Option [Member]
Expenses on stock options				800	$ 800	$ 5,200	$ 2,300
Total unrecognized compensation expense related to unvested time-based restricted stock				$ 2,400		$ 2,400
Weighted-average period of recognition for total unrecognized compensation expense related to unvested time-based restricted stock						1 year 3 months 18 days
Milestone Based Restricted Stock [Member]
Restricted stock outstanding				2,860,511		2,860,511
2017 ATM [Member]
Gross proceeds			$ 74,200
Share price in offering			$ 28.73
Proceeds from offering	$ 72,900
2020 ATM [Member]
Percent of commission rate										3.00%
Shares sold			2,582,678			5,945,608
Gross proceeds						$ 113,300
Share price in offering				$ 19.06		$ 19.06
Proceeds from offering						$ 111,300
Underwritten Public Offering [Member]
Share Price		$ 18
Proceeds from offering		$ 165,100
Issuance of common stock (net of offering costs) (in shares)		8,500,000
Payments of underwriting discounts and offering expenses		$ 10,800
Over-Allotment Option [Member]
Issuance of common stock (net of offering costs) (in shares)		1,275,000
Amended and Restated 2012 Incentive Plan [Member]
Number of additional shares that may be issued				5,054,913		5,054,913
Amended and Restated 2012 Incentive Plan [Member] | Unvested Restricted stock [Member]
Restricted stock outstanding				9,909,709		9,909,709
Amended and Restated 2012 Incentive Plan [Member] | Employee Stock Option [Member]
Restricted stock outstanding				2,529,133		2,529,133