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STOCKHOLDERS' EQUITY - SALES AGREEMENTS (Details) - USD ($)			1 Months Ended	12 Months Ended
	May 18, 2018	Aug. 28, 2015	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
At Market Issuance Sales Agreements [Line Items]
Net cash proceeds from issuance of common stock after deducting sales commissions and offering costs				$ 19,295,000	$ 86,017,000	$ 1,060,000
2015 Sales Agreement
At Market Issuance Sales Agreements [Line Items]
Aggregate offering price of common stock		$ 50,000,000
Maximum commission rate (as a percent)		3.00%
Issuance of common stock in connection with at market offering (in shares)			614,230		13,195,106
Net cash proceeds from issuance of common stock after deducting sales commissions and offering costs			$ 1,060,000		$ 47,651,000
2018 Sales Agreement
At Market Issuance Sales Agreements [Line Items]
Aggregate offering price of common stock	$ 100,000,000
Maximum commission rate (as a percent)	3.00%
Issuance of common stock in connection with at market offering (in shares)				13,214,867	10,083,079
Net cash proceeds from issuance of common stock after deducting sales commissions and offering costs				$ 19,295,000	$ 38,366,000