Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Balance at Dec. 31, 2017	$ 37,225	$ 34	$ 248,278	$ (211,087)
Balance (in shares) at Dec. 31, 2017		34,210
Increase (Decrease) in Stockholders' Equity [Rollforward]
Issuance of common stock on exercise of stock options and vesting of restricted stock units	108	$ 0	108
Issuance of common stock on exercise of stock options and vesting of restricted stock units (in shares)		44
Issuance of common stock under employee stock purchase plan (ESPP)	397		397
Issuance of common stock on employee stock purchase plan (ESPP) (in shares)		73
Tax portion of vested restricted stock units	(30)		(30)
Stock-based compensation expense (employee)	1,083		1,083
Stock-based compensation expense (non-employee)	1		1
Stock-based compensation expense (ESPP)	91		91
Legal settlement from short-swing profits, net of tax	310		310
Net loss	(9,177)			(9,177)
Comprehensive loss	(9,177)			(9,177)
Balance at Mar. 31, 2018	30,008	$ 34	250,238	(220,264)
Balance (in shares) at Mar. 31, 2018		34,327
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 on employee stock purchase plan (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)			(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