Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Total
Balance at Dec. 31, 2017	$ 17	$ 84,202	$ (51)	$ (38,173)	$ 45,995
Balance, shares at Dec. 31, 2017	16,819
Issuance of shares upon exercise of stock options		164			164
Issuance of shares upon exercise of stock options, shares	55
Issuance of shares under employee stock purchase plan		188			188
Issuance of shares under employee stock purchase plan, shares	12
Stock-based compensation expense		6,595			6,595
Tax payments related to shares withheld for vested restricted stock units		(115)			(115)
Unrealized gain on available-for-sale securities			51		51
Net loss				(17,831)	(17,831)
Balance at Jun. 30, 2018	$ 17	91,034		(56,004)	35,047
Balance, shares at Jun. 30, 2018	16,886
Balance at Mar. 31, 2018	$ 17	87,922	(3)	(46,841)	41,095
Balance, shares at Mar. 31, 2018	16,869
Issuance of shares upon exercise of stock options		52			52
Issuance of shares upon exercise of stock options, shares	17
Stock-based compensation expense		3,175			3,175
Tax payments related to shares withheld for vested restricted stock units		(115)			(115)
Unrealized gain on available-for-sale securities			3		3
Net loss				(9,163)	(9,163)
Balance at Jun. 30, 2018	$ 17	91,034		(56,004)	35,047
Balance, shares at Jun. 30, 2018	16,886
Balance at Dec. 31, 2018	$ 21	142,032	(1)	(75,718)	$ 66,334
Balance, shares at Dec. 31, 2018	20,593				20,593
Issuance of shares upon exercise of stock options		272			$ 272
Issuance of shares upon exercise of stock options, shares	99
Issuance of shares under employee stock purchase plan		222			222
Issuance of shares under employee stock purchase plan, shares	18
Issuance of shares upon vesting of restriced stock units
Issuance of shares upon vesting of restriced stock units, shares	58
Stock-based compensation expense		5,060			5,060
Tax payments related to shares withheld for vested restricted stock units		(613)			(613)
Unrealized gain on available-for-sale securities			23		23
Net loss				(21,437)	(21,437)
Balance at Jun. 30, 2019	$ 21	146,973	22	(97,155)	$ 49,861
Balance, shares at Jun. 30, 2019	20,768				20,768
Balance at Mar. 31, 2019	$ 21	144,887	2	(85,796)	$ 59,114
Balance, shares at Mar. 31, 2019	20,710
Issuance of shares upon vesting of restriced stock units
Issuance of shares upon vesting of restriced stock units, shares	58
Stock-based compensation expense		2,699			2,699
Tax payments related to shares withheld for vested restricted stock units		(613)			(613)
Unrealized gain on available-for-sale securities			20		20
Net loss				(11,359)	(11,359)
Balance at Jun. 30, 2019	$ 21	$ 146,973	$ 22	$ (97,155)	$ 49,861
Balance, shares at Jun. 30, 2019	20,768				20,768