Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		81,468,891
Beginning balance at Dec. 31, 2017	$ 235,430	$ 81	$ 364,352	$ (129,003)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to exercise of stock options (in shares)		1,198,732
Issuance of common stock pursuant to exercise of stock options	6,002	$ 2	6,000
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		7,532
Issuance of common stock pursuant to settlement of restricted stock units	0
Common stock surrendered for employees' tax liability upon settlement of restricted stock units (in shares)		(2,563)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(58)		(58)
Stock-based compensation	4,320		4,320
Net loss	(36,439)			(36,439)
Ending balance (in shares) at Mar. 31, 2018		82,672,592
Ending balance at Mar. 31, 2018	$ 209,255	$ 83	374,614	(165,442)
Beginning balance (in shares) at Dec. 31, 2018	90,151,341	90,151,341
Beginning balance at Dec. 31, 2018	$ 371,938	$ 90	542,829	(170,981)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to exercise of stock options (in shares)	679,495	679,495
Issuance of common stock pursuant to exercise of stock options	$ 3,732	$ 1	3,731
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		139,889
Issuance of common stock pursuant to settlement of restricted stock units	0
Common stock surrendered for employees' tax liability upon settlement of restricted stock units (in shares)		(44,476)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(818)		(818)
Stock-based compensation	6,676		6,676
Other comprehensive income	1			1
Net loss	$ (67,180)			(67,180)
Ending balance (in shares) at Mar. 31, 2019	90,926,249	90,926,249
Ending balance at Mar. 31, 2019	$ 314,349	$ 91	$ 552,418	$ (238,160)