Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Balance at Dec. 31, 2018	$ 79,755	$ 41	$ 313,800	$ (234,086)
Balance (in shares) at Dec. 31, 2018		40,863
Increase (Decrease) in Stockholders' Equity [Rollforward]
Issuance of common stock on exercise of stock options and vesting of restricted stock units	4,240	$ 1	4,239
Issuance of common stock on exercise of stock options and vesting of restricted stock units (in shares)		566
Issuance of common stock under employee stock purchase plan (ESPP)	491		491
Issuance of common stock under ESPP (in shares)		80
Tax portion of vested restricted stock units	(556)		(556)
Stock-based compensation expense (employee)	1,598		1,598
Stock-based compensation expense (non-employee)	20		20
Stock-based compensation expense (ESPP)	141		141
Net loss	(1,917)
Comprehensive loss	(1,917)			(1,917)
Balance at Mar. 31, 2019	83,772	$ 42	319,733	(236,003)
Balance (in shares) at Mar. 31, 2019		41,509
Balance at Dec. 31, 2019	239,455	$ 50	486,090	(246,685)
Balance (in shares) at Dec. 31, 2019		49,625
Increase (Decrease) in Stockholders' Equity [Rollforward]
Issuance of common stock on exercise of stock options and vesting of restricted stock units	981		981
Issuance of common stock on exercise of stock options and vesting of restricted stock units (in shares)		314
Issuance of common stock under employee stock purchase plan (ESPP)	1,101		1,101
Issuance of common stock under ESPP (in shares)		61
Tax portion of vested restricted stock units	(2,304)		(2,304)
Stock-based compensation expense (employee)	2,551		2,551
Stock-based compensation expense (ESPP)	354		354
Net loss	(11,716)
Comprehensive loss	(11,716)			(11,716)
Balance at Mar. 31, 2020	$ 230,422	$ 50	$ 488,773	$ (258,401)
Balance (in shares) at Mar. 31, 2020		50,000