Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Total
Balance at Dec. 31, 2017	$ 6	$ 472,552	$ (766)	$ (446,375)	$ 25,417
Balance (in shares) at Dec. 31, 2017	54,040
Issuance of common stock upon exercise of stock options		564			564
Issuance of common stock upon exercise of stock options (in shares)	145
Vesting of restricted stock (in shares)	66
Stock based compensation		3,154			3,154
Unrealized gain on available-for-sale securities			(138)		(138)
Net loss				(32,873)	(32,873)
Balance at Mar. 31, 2018	$ 6	476,270	(904)	(479,248)	(3,876)
Balance (in shares) at Mar. 31, 2018	54,251
Balance at Dec. 31, 2018	$ 7	607,236	(552)	(574,529)	$ 32,162
Balance (in shares) at Dec. 31, 2018	62,083				62,083
Cumulative-effect adjustment upon adoption of ASU 2018-07		(185)		185
Issuance of common stock upon exercise of stock options		2,578			$ 2,578
Issuance of common stock upon exercise of stock options (in shares)	1,070
Vesting of restricted stock (in shares)	187
Stock based compensation		4,051			4,051
Unrealized gain on available-for-sale securities			286		286
Net loss				(34,091)	(34,091)
Balance at Mar. 31, 2019	$ 7	$ 613,680	$ (266)	$ (608,435)	$ 4,986
Balance (in shares) at Mar. 31, 2019	63,340				63,340