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STOCKHOLDERS' EQUITY - AT MARKET ISSUANCE SALES AGREEMENT (Details) - USD ($)			3 Months Ended			6 Months Ended
	May 18, 2018	Aug. 28, 2015	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
At Market Issuance Sales Agreements [Line Items]
Net cash proceeds from issuance of common stock after deducting sales commissions and other offering costs			$ 101,000	$ 82,306,000	$ 1,553,000
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)							13,195,106
Net cash proceeds from issuance of common stock after deducting sales commissions and other offering costs							$ 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)			108,386	9,447,026		108,386	9,447,026
Net cash proceeds from issuance of common stock after deducting sales commissions and other offering costs			$ 101,000	$ 36,208,000		$ 101,000	$ 36,208,000