STOCK OPTIONS (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended	12 Months Ended			18 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Jun. 16, 2016	Apr. 30, 2015	Mar. 31, 2015
Stock options
Maximum aggregate number of shares reserved for issuance during term of employee stock purchase plan		6,035,394	6,035,394			6,035,394
Stock-based compensation (in dollars)			$ 4,106	$ 5,063	$ 4,666
Stock Options
Stock options
Options to purchase common stock outstanding (in shares)		3,369,245	3,369,245	3,193,785		3,369,245
Granted (in shares)			1,439,463
Expected term			10 years
Vesting period			48 months
2007 Plan
Stock options
Options to purchase common stock outstanding (in shares)		46,476	46,476			46,476
Shares available for future grants		0	0			0
Expected term			10 years
2010 Plan
Stock options
Options to purchase common stock outstanding (in shares)		1,444,644	1,444,644			1,444,644
Shares available for future grants							322,355
Granted (in shares)						0
Expected term			10 years
2015 Plan
Stock options
Options to purchase common stock outstanding (in shares)		1,878,125	1,878,125			1,878,125
Shares authorized for issuance		4,322,355	4,322,355			4,322,355		4,000,000
Expected term			10 years
Employee Stock Purchase Plan
Stock options
Shares authorized for issuance									187,500
Percentage of outstanding shares reserved for future issuance			1.00%
Increase in shares of common stock reserved for issuance (in shares)			50,000
Maximum aggregate number of shares reserved for issuance during term of employee stock purchase plan		1,250,000	1,250,000			1,250,000
Offering Period			6 months
Stock issued under ESPP (in shares)	4,691
Stock-based compensation (in dollars)			$ 14	$ 46
Common Stock
Stock options
Stock issued under ESPP (in shares)			17,750	16,729
Accrued expenses. | Employee Stock Purchase Plan
Stock options
Employee payroll deductions (in dollars)		$ 3