EQUITY - Changes in Shareholders' Equity (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Oct. 31, 2016	Jan. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Balance at beginning of period					$ 134,908
Exercise of common stock options					$ 428
Exercise of common stock options, shares					131,755
Issuance for employee stock purchase plan					$ 1,141
Issuance for employee stock purchase plan, shares					110,841
Shares withheld for employee taxes upon vesting of restricted stock units					$ (68)
Issuance of common stock resulting from at-the-market transactions, net of issuance costs of $507					$ 11,455
Issuance of common stock resulting from at-the-market transactions, net of issuance costs of $507 (in shares)	5,750,000	2,750,000			1,148,466
Stock-based compensation expense					$ 5,867
Net loss			$ (13,263)	$ (26,444)	(57,462)	$ (66,617)
Balance at end of period			96,269		96,269
Issuance costs					507	$ 526
Common Stock
Balance at beginning of period					$ 29
Balance at beginning of year, shares					29,364,100
Exercise of common stock options					$ 1
Exercise of common stock options, shares					131,755
Issuance for employee stock purchase plan, shares					110,841
Shares withheld for employee taxes upon vesting of restricted stock units, shares					(4,819)
Issuance of common stock resulting from at-the-market transactions, net of issuance costs of $507					$ 1
Issuance of common stock resulting from at-the-market transactions, net of issuance costs of $507 (in shares)					1,148,466
Vesting of restricted stock units, shares					14,757
Balance at end of period			$ 31		$ 31
Balance at end of year, shares			30,765,100		30,765,100
Additional Paid-In Capital
Balance at beginning of period					$ 358,063
Exercise of common stock options					427
Issuance for employee stock purchase plan					1,141
Shares withheld for employee taxes upon vesting of restricted stock units					(68)
Issuance of common stock resulting from at-the-market transactions, net of issuance costs of $507					11,454
Stock-based compensation expense					5,867
Balance at end of period			$ 376,884		376,884
Accumulated Deficit
Balance at beginning of period					(223,184)
Net loss					(57,462)
Balance at end of period			$ (280,646)		$ (280,646)