Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2017		37,802,556
Beginning balance at Dec. 31, 2017	$ 154,496	$ 38	$ 365,838	$ (211,269)	$ (111)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock compensation expense	2,050		2,050
Exercise of common stock options (in shares)		106,502
Exercise of common stock options	130		130
Foreign currency translation adjustment	7				7
Net loss	(30,572)			(30,572)
Ending balance (in shares) at Mar. 31, 2018		37,909,058
Ending balance at Mar. 31, 2018	126,111	$ 38	368,018	(241,841)	(104)
Beginning balance (in shares) at Dec. 31, 2017		37,802,556
Beginning balance at Dec. 31, 2017	154,496	$ 38	365,838	(211,269)	(111)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(55,154)
Ending balance (in shares) at Jun. 30, 2018		41,121,557
Ending balance at Jun. 30, 2018	164,333	$ 41	430,803	(266,422)	(89)
Beginning balance (in shares) at Mar. 31, 2018		37,909,058
Beginning balance at Mar. 31, 2018	126,111	$ 38	368,018	(241,841)	(104)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock compensation expense	2,135		2,135
Sale of common stock, net of issuance costs (in shares)		2,875,000
Sale of common stock, net of issuance costs	59,926	$ 3	59,923
Exercise of common stock options (in shares)		330,401
Exercise of common stock options	727		727
Exercise of warrants (in shares)		7,098
Exercise of warrants	0
Foreign currency translation adjustment	15				15
Net loss	(24,581)			(24,581)
Ending balance (in shares) at Jun. 30, 2018		41,121,557
Ending balance at Jun. 30, 2018	164,333	$ 41	430,803	(266,422)	(89)
Beginning balance (in shares) at Dec. 31, 2018		41,227,530
Beginning balance at Dec. 31, 2018	118,611	$ 41	436,554	(317,927)	(57)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock compensation expense	2,425		2,425
Exercise of common stock options (in shares)		5,000
Exercise of common stock options	15		15
Issuance of common stock under employee stock purchase plan (in shares)		31,892
Issuance of common stock under employee stock purchase plan	173		173
Foreign currency translation adjustment	3				3
Net loss	(28,874)			(28,874)
Ending balance (in shares) at Mar. 31, 2019		41,264,422.000
Ending balance at Mar. 31, 2019	92,353	$ 41	439,167	(346,801)	(54)
Beginning balance (in shares) at Dec. 31, 2018		41,227,530
Beginning balance at Dec. 31, 2018	118,611	$ 41	436,554	(317,927)	(57)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(56,251)
Ending balance (in shares) at Jun. 30, 2019		41,430,918.000
Ending balance at Jun. 30, 2019	68,472	$ 41	442,676	(374,178)	(67)
Beginning balance (in shares) at Mar. 31, 2019		41,264,422.000
Beginning balance at Mar. 31, 2019	92,353	$ 41	439,167	(346,801)	(54)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock compensation expense	2,716		2,716
Exercise of common stock options (in shares)		88,587
Exercise of common stock options	354		354
Issuance of common stock under employee stock purchase plan (in shares)		77,909
Issuance of common stock under employee stock purchase plan	439		439
Foreign currency translation adjustment	(13)				(13)
Net loss	(27,377)			(27,377)
Ending balance (in shares) at Jun. 30, 2019		41,430,918.000
Ending balance at Jun. 30, 2019	$ 68,472	$ 41	$ 442,676	$ (374,178)	$ (67)