CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($)	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Loss	Accumulated Deficit	Total
Balance at Dec. 31, 2017	$ 41,000	$ 202,790,000	$ 29,231	$ (96,000)	$ (144,727,000)	$ 58,008,000
Balance (in shares) at Dec. 31, 2017	40,549,936
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		1,127,000				1,127,000
Unrealized gain (loss) on investments				(11,000)		(11,000)
Net loss					(5,999,000)	(5,999,000)
Balance at Mar. 31, 2018	$ 41,000	203,917,000	29,231	(107,000)	(150,726,000)	53,125,000
Balance (in shares) at Mar. 31, 2018	40,549,936
Balance at Dec. 31, 2017	$ 41,000	202,790,000	29,231	(96,000)	(144,727,000)	58,008,000
Balance (in shares) at Dec. 31, 2017	40,549,936
Increase (Decrease) in Stockholders' Equity (Deficit)
Net loss						(26,614,000)
Balance at Sep. 30, 2018	$ 41,000	206,450,000	29,231	(28,000)	(171,341,000)	35,122,000
Balance (in shares) at Sep. 30, 2018	40,554,244
Balance at Mar. 31, 2018	$ 41,000	203,917,000	29,231	(107,000)	(150,726,000)	53,125,000
Balance (in shares) at Mar. 31, 2018	40,549,936
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		1,348,000				1,348,000
Exercise of stock options		14,000				14,000
Exercise of stock options (in shares)	12,308
Unrealized gain (loss) on investments				38,000		38,000
Net loss					(9,505,000)	(9,505,000)
Balance at Jun. 30, 2018	$ 41,000	205,279,000	29,231	(69,000)	(160,231,000)	45,020,000
Balance (in shares) at Jun. 30, 2018	40,562,244
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		1,171,000				1,171,000
Forfeiture of restricted stock (in shares)	(8,000)
Unrealized gain (loss) on investments				41,000		41,000
Net loss					(11,110,000)	(11,110,000)
Balance at Sep. 30, 2018	$ 41,000	206,450,000	29,231	(28,000)	(171,341,000)	35,122,000
Balance (in shares) at Sep. 30, 2018	40,554,244
Balance at Dec. 31, 2018	$ 53,000	249,727,000	29,231	(2,000)	(181,453,000)	68,325,000
Balance (in shares) at Dec. 31, 2018	52,548,244
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		1,836,000				1,836,000
Exercise of stock options		65,000				65,000
Exercise of stock options (in shares)	55,812
Forfeiture of restricted stock (in shares)	(20,200)
Unrealized gain (loss) on investments				2,000		2,000
Net loss					(12,483,000)	(12,483,000)
Balance at Mar. 31, 2019	$ 53,000	251,628,000	29,231		(193,936,000)	57,745,000
Balance (in shares) at Mar. 31, 2019	52,583,856
Balance at Dec. 31, 2018	$ 53,000	249,727,000	29,231	(2,000)	(181,453,000)	68,325,000
Balance (in shares) at Dec. 31, 2018	52,548,244
Increase (Decrease) in Stockholders' Equity (Deficit)
Net loss						(38,712,000)
Balance at Sep. 30, 2019	$ 53,000	254,182,000	29,231	1,000	(220,165,000)	34,071,000
Balance (in shares) at Sep. 30, 2019	52,608,064
Balance at Mar. 31, 2019	$ 53,000	251,628,000	29,231		(193,936,000)	57,745,000
Balance (in shares) at Mar. 31, 2019	52,583,856
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		1,263,000				1,263,000
Net loss					(12,423,000)	(12,423,000)
Balance at Jun. 30, 2019	$ 53,000	252,891,000	29,231		(206,359,000)	46,585,000
Balance (in shares) at Jun. 30, 2019	52,583,856
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation expense		1,339,000				1,339,000
Exercise of stock options		32,000				32,000
Exercise of stock options (in shares)	24,208
Financing costs under equity distribution agreement		(80,000)				(80,000)
Unrealized gain (loss) on investments				1,000		1,000
Net loss					(13,806,000)	(13,806,000)
Balance at Sep. 30, 2019	$ 53,000	$ 254,182,000	$ 29,231	$ 1,000	$ (220,165,000)	$ 34,071,000
Balance (in shares) at Sep. 30, 2019	52,608,064