Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	AOCI Attributable to Parent	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2018		45,563,693
Beginning balance at Dec. 31, 2018	$ 60,632	$ 5	$ 1,165,003	$ (755)	$ (1,103,621)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(42,760)				(42,760)
Unrealized gain from available-for-sale securities	474			474
Vesting of restricted shares (in shares)		73,113
Vesting of restricted shares	0
Exercise of options (in shares)		185,438
Exercise of options	1,517		1,517
Exercise of warrants (in shares)		81,104
Exercise of warrants	1,000		1,000
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		63,732
Issuance of common stock upon purchase by employee stock purchase plan	1,027		1,027
Share-based compensation expense	6,114		6,114
Ending balance (in shares) at Mar. 31, 2019		45,967,080
Ending balance at Mar. 31, 2019	28,004	$ 5	1,174,661	(281)	(1,146,381)
Beginning balance (in shares) at Dec. 31, 2019		46,189,870
Beginning balance at Dec. 31, 2019	(42,279)	$ 5	1,194,327	3	(1,236,614)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(37,654)
Unrealized gain from available-for-sale securities	(669)			(669)
Vesting of restricted shares (in shares)		142,270
Vesting of restricted shares	0
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		55,297
Issuance of common stock upon purchase by employee stock purchase plan	990		990
Share-based compensation expense	5,459		5,459
Ending balance (in shares) at Mar. 31, 2020		46,387,437
Ending balance at Mar. 31, 2020	$ (74,153)	$ 5	$ 1,200,776	$ (666)	$ (1,274,268)