SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) - Summary of Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Revenue	$ 880	$ 695	$ 2,514	$ 372	$ 476	$ 944	$ 2,182	$ 4,784	$ 4,461	$ 8,386
Research and development expenses	(8,417)	(9,027)	(8,599)	(8,751)	(8,234)	(7,399)	(8,375)	(6,413)	(34,794)	(30,421)
General and administrative expenses	(2,534)	(2,898)	(2,857)	(3,357)	(2,695)	(2,696)	(2,668)	(2,990)	(11,646)	(11,049)
Other income (expense), net	(47)	0	46	107	136	140	147	(42)
Net loss	(10,118)	(11,230)	(8,896)	(11,629)	(10,317)	(9,011)	(8,714)	(4,661)	(41,873)	(32,703)
Comprehensive loss	$ (10,121)	$ (11,238)	$ (8,887)	$ (11,624)	$ (10,314)	$ (9,021)	$ (8,710)	$ (4,661)	$ (41,873)	$ (32,703)
Net loss per share, basic and diluted (in dollars per share)	$ (0.31)	$ (0.36)	$ (0.29)	$ (0.38)	$ (0.33)	$ (0.29)	$ (0.29)	$ (0.18)	$ (1.34)	$ (1.10)
Restructuring charges	$ 900	$ 1,100							$ 2,034
Research and development
Restructuring Cost and Reserve [Line Items]
Restructuring charges	800	900							1,700
General and administrative
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 100	$ 200							$ 300