Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	AOCI Attributable to Parent	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		44,616,586
Beginning balance at Dec. 31, 2017	$ 242,036	$ 4	$ 1,124,630	$ (314)	$ (882,284)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(61,555)				(61,555)
Unrealized loss from available-for-sale securities	(1,169)			(1,169)
Vesting of restricted shares (in shares)		16,111
Exercise of options (in shares)		413,686
Exercise of options	6,576	$ 1	6,575
Exercise of warrants (in shares)		113,314
Share-based compensation expense related to share-based awards for employee stock purchase plan	162		162
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		54,690
Issuance of common stock upon purchase by employee stock purchase plan	1,741		1,741
Share-based compensation expense	7,387		7,387
Ending balance (in shares) at Mar. 31, 2018		45,214,387
Ending balance at Mar. 31, 2018	195,178	$ 5	1,140,495	(1,483)	(943,839)
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 loss from available-for-sale securities	474			474
Vesting of restricted shares (in shares)		73,113
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
Share-based compensation expense related to share-based awards for employee stock purchase plan	181		181
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	5,933		5,933
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)