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 at Mar. 31, 2018	126,111	$ 38	368,018	(241,841)	(104)
Ending balance (in shares) at Mar. 31, 2018		37,909,058
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	(80,277)
Ending balance at Sep. 30, 2018	142,732	$ 41	434,312	(291,545)	(76)
Ending balance (in shares) at Sep. 30, 2018		41,227,530
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 at Jun. 30, 2018	164,333	$ 41	430,803	(266,422)	(89)
Ending balance (in shares) at Jun. 30, 2018		41,121,557
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock compensation expense	2,209		2,209
Exercise of common stock options (in shares)		45,287
Exercise of common stock options	561		561
Issuance of common stock under employee stock purchase plan (in shares)		53,137
Issuance of common stock under employee stock purchase plan	739		739
Exercise of warrants (in shares)		7,549
Exercise of warrants	0
Foreign currency translation adjustment	13				13
Net loss	(25,123)			(25,123)
Ending balance at Sep. 30, 2018	142,732	$ 41	434,312	(291,545)	(76)
Ending balance (in shares) at Sep. 30, 2018		41,227,530
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 at Mar. 31, 2019	92,353	$ 41	439,167	(346,801)	(54)
Ending balance (in shares) at Mar. 31, 2019		41,264,422.000
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	(85,058)
Ending balance at Sep. 30, 2019	44,518	$ 41	447,541	(402,985)	(79)
Ending balance (in shares) at Sep. 30, 2019		41,488,370
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	(27,376)			(27,376)
Ending balance at Jun. 30, 2019	68,473	$ 41	442,676	(374,177)	(67)
Ending balance (in shares) at Jun. 30, 2019		41,430,918.000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock compensation expense	2,469		2,469
Exercise of common stock options (in shares)		57,452
Exercise of common stock options	172		172
Issuance of warrants	2,224		2,224
Foreign currency translation adjustment	(12)				(12)
Net loss	(28,808)			(28,808)
Ending balance at Sep. 30, 2019	$ 44,518	$ 41	$ 447,541	$ (402,985)	$ (79)
Ending balance (in shares) at Sep. 30, 2019		41,488,370