Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock Warrants [Member]	Common Stock	Additional Paid-in Capital [Member] Warrants [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member] Warrants [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member] Warrants [Member]	Accumulated Deficit [Member]	Warrants [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				423						423
Issuance of shares upon exercise of stock options, shares	24	117
Issuance of shares under employee stock purchase plan				327						327
Issuance of shares under employee stock purchase plan, shares		24
Stock-based compensation expense				9,988						9,988
Tax payments related to shares withheld for vested restricted stock units				(115)						(115)
Unrealized gains (losses) on available-for-sale securities						48				48
Net loss								(28,592)		(28,592)
Balance at Sep. 30, 2018		$ 17		94,825		(3)		(66,765)		28,074
Balance, shares at Sep. 30, 2018		16,984
Balance at Jun. 30, 2018		$ 17		91,034				(56,004)		35,047
Balance, shares at Jun. 30, 2018		16,886
Issuance of shares upon exercise of stock options				258						258
Issuance of shares upon exercise of stock options, shares	24	62
Issuance of shares under employee stock purchase plan				139						139
Issuance of shares under employee stock purchase plan, shares		12
Stock-based compensation expense				3,394						3,394
Unrealized gains (losses) on available-for-sale securities						(3)				(3)
Net loss								(10,761)		(10,761)
Balance at Sep. 30, 2018		$ 17		94,825		(3)		(66,765)		28,074
Balance, shares at Sep. 30, 2018		16,984
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				423						423
Issuance of shares under employee stock purchase plan, shares		38
Issuance of shares upon vesting of restriced stock units
Issuance of shares upon vesting of restriced stock units, shares		58
Stock-based compensation expense				7,739						7,739
Tax payments related to shares withheld for vested restricted stock units				(613)						(613)
Unrealized gains (losses) on available-for-sale securities						9				9
Net loss								(33,183)		(33,183)
Balance at Sep. 30, 2019		$ 21		149,853		8		(108,901)		$ 40,981
Balance, shares at Sep. 30, 2019		20,788								20,788
Balance at Jun. 30, 2019		$ 21		146,973		22		(97,155)		$ 49,861
Balance, shares at Jun. 30, 2019		20,768
Issuance of shares under employee stock purchase plan				201						201
Issuance of shares under employee stock purchase plan, shares		20
Stock-based compensation expense				2,679						2,679
Unrealized gains (losses) on available-for-sale securities						(14)				(14)
Net loss								(11,746)		(11,746)
Balance at Sep. 30, 2019		$ 21		$ 149,853		$ 8		$ (108,901)		$ 40,981
Balance, shares at Sep. 30, 2019		20,788								20,788