Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income
Beginning balance (in shares) at Dec. 31, 2017		81,468,891
Beginning balance at Dec. 31, 2017	$ 235,430	$ 81	$ 364,352	$ (129,003)	$ 0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to employee stock purchase program (in shares)		187,076
Issuance of common stock pursuant to employee stock purchase program	3,671		3,671
Issuance of common stock pursuant to exercise of stock options (in shares)		2,108,485
Issuance of common stock pursuant to exercise of stock options	10,753	$ 3	10,750
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		31,022
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)		(10,223)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(227)		(227)
Stock-based compensation	9,218		9,218
Net loss	(33,232)
Ending balance (in shares) at Jun. 30, 2018		83,785,251
Ending balance at Jun. 30, 2018	225,613	$ 84	387,764	(162,235)	0
Beginning balance (in shares) at Mar. 31, 2018		82,672,592
Beginning balance at Mar. 31, 2018	209,255	$ 83	374,614	(165,442)	0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to employee stock purchase program (in shares)		187,076
Issuance of common stock pursuant to employee stock purchase program	3,671		3,671
Issuance of common stock pursuant to exercise of stock options (in shares)		909,753
Issuance of common stock pursuant to exercise of stock options	4,750	$ 1	4,749
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		23,490
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)		(7,660)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(167)		(167)
Stock-based compensation	4,897		4,897
Net loss	3,207
Ending balance (in shares) at Jun. 30, 2018		83,785,251
Ending balance at Jun. 30, 2018	$ 225,613	$ 84	387,764	(162,235)	0
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)	0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to employee stock purchase program (in shares)		262,110
Issuance of common stock pursuant to employee stock purchase program	$ 3,246		3,246
Issuance of common stock pursuant to exercise of stock options (in shares)	1,099,599	1,099,599
Issuance of common stock pursuant to exercise of stock options	$ 5,770	$ 2	5,768
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		366,915
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)		(102,428)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(1,792)		(1,792)
Stock-based compensation	12,843		12,843
Other comprehensive income	44				44
Net loss	$ (79,804)
Ending balance (in shares) at Jun. 30, 2019	91,777,537	91,777,537
Ending balance at Jun. 30, 2019	$ 312,245	$ 92	562,894	(250,785)	44
Beginning balance (in shares) at Mar. 31, 2019		90,926,249
Beginning balance at Mar. 31, 2019	314,349	$ 91	552,418	(238,160)	1
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to employee stock purchase program (in shares)		262,110
Issuance of common stock pursuant to employee stock purchase program	3,246		3,246
Issuance of common stock pursuant to exercise of stock options (in shares)		420,104
Issuance of common stock pursuant to exercise of stock options	2,039	$ 1	2,038
Issuance of common stock pursuant to settlement of restricted stock units (in shares)		227,026
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)		(57,952)
Common stock surrendered for employees' tax liability upon settlement of restricted stock units	(975)		(975)
Stock-based compensation	6,167		6,167
Other comprehensive income	43				43
Net loss	$ (12,625)			(12,625)
Ending balance (in shares) at Jun. 30, 2019	91,777,537	91,777,537
Ending balance at Jun. 30, 2019	$ 312,245	$ 92	$ 562,894	$ (250,785)	$ 44