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STOCKHOLDERS' EQUITY - AT MARKET ISSUANCE SALES AGREEMENTS (Details) - USD ($)			3 Months Ended	6 Months Ended
	May 18, 2018	Aug. 28, 2015	Jun. 30, 2018	Jun. 30, 2018
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 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)			9,447,026
Net cash proceeds from issuance of common stock after deducting sales commissions and offering costs			$ 36,208,000

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At Market Issuance Sales Agreements.

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Represents the aggregate offering price for which the entity may elect to issue and sell shares of its common stock under the sales agreement.

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Represents the percentage of maximum commission rate, based on gross proceeds of sale price per share of common stock sold through sales agent under the sales agreement.

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Number of new stock issued during the period.

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