Condensed Consolidated Statement of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2017	$ 60,699	$ 40	$ 324,376	$ (263,571)	$ (146)
Beginning balance, shares at Dec. 31, 2017		40,571,015
Issuance of common stock upon exercise of stock options, value	32		32
Issuance of common stock upon exercise of stock options, shares		48,053
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		51,500
Repurchase of common stock for employee tax withholdings, value	(197)		(197)
Repurchase of common stock for employee tax withholdings, shares		(17,900)
Stock-based compensation expense	4,236		4,236
Unrealized gain on investments	40				40
Adoption of new revenue standard (ASC 606)	(26,857)			(26,857)
Net loss	(27,919)			(27,919)
Ending balance at Mar. 31, 2018	10,034	$ 40	328,447	(318,347)	(106)
Ending balance, shares at Mar. 31, 2018		40,652,668
Beginning balance at Dec. 31, 2017	60,699	$ 40	324,376	(263,571)	(146)
Beginning balance, shares at Dec. 31, 2017		40,571,015
Net loss	(77,655)
Ending balance at Sep. 30, 2018	(30,565)	$ 40	337,486	(368,083)	(8)
Ending balance, shares at Sep. 30, 2018		40,844,455
Beginning balance at Mar. 31, 2018	10,034	$ 40	328,447	(318,347)	(106)
Beginning balance, shares at Mar. 31, 2018		40,652,668
Issuance of common stock upon exercise of stock options, value	64		64
Issuance of common stock upon exercise of stock options, shares		92,013
Issuance of common stock upon vesting of RSUs, net of tax withholdings, value	27		27
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		10,000
Stock-based compensation expense	4,334		4,334
Unrealized gain on investments	77				77
Net loss	(27,787)			(27,787)
Ending balance at Jun. 30, 2018	(13,251)	$ 40	332,872	(346,134)	(29)
Ending balance, shares at Jun. 30, 2018		40,754,681
Issuance of common stock upon exercise of stock options, value	40		40
Issuance of common stock upon exercise of stock options, shares		56,442
Issuance of common stock under ESPP plan, value	257		257
Issuance of common stock under ESPP plan, shares		33,332
Stock-based compensation expense	4,317		4,317
Unrealized gain on investments	21				21
Net loss	(21,949)			(21,949)
Ending balance at Sep. 30, 2018	(30,565)	$ 40	337,486	(368,083)	(8)
Ending balance, shares at Sep. 30, 2018		40,844,455
Beginning balance at Dec. 31, 2018	$ (48,045)	$ 41	341,284	(389,370)
Beginning balance, shares at Dec. 31, 2018	40,936,735	40,936,735
Issuance of common stock upon exercise of stock options, value	$ 120		120
Issuance of common stock upon exercise of stock options, shares		38,125
Issuance of common stock upon vesting of RSUs, net of tax withholdings, value	153		153
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		73,500
Issuance of common stock under ESPP plan, value	207		207
Issuance of common stock under ESPP plan, shares		46,472
Stock-based compensation expense	2,065		2,065
Net loss	(24,333)			(24,333)
Ending balance at Mar. 31, 2019	(69,833)	$ 41	343,829	(413,703)
Ending balance, shares at Mar. 31, 2019		41,094,832
Beginning balance at Dec. 31, 2018	$ (48,045)	$ 41	341,284	(389,370)
Beginning balance, shares at Dec. 31, 2018	40,936,735	40,936,735
Issuance of common stock upon exercise of stock options, shares	90,125
Net loss	$ (51,501)
Ending balance at Sep. 30, 2019	$ (32,219)	$ 70	408,575	(440,871)	7
Ending balance, shares at Sep. 30, 2019	69,993,952	69,993,952
Beginning balance at Mar. 31, 2019	$ (69,833)	$ 41	343,829	(413,703)
Beginning balance, shares at Mar. 31, 2019		41,094,832
Issuance of common stock from public offering, net of commissions, underwriting, value	60,527	$ 29	60,498
Issuance of common stock from public offering, net of commissions, underwriting, shares		28,818,578
Stock-based compensation expense	2,097		2,097
Net loss	(10,759)			(10,759)
Ending balance at Jun. 30, 2019	(17,968)	$ 70	406,424	(424,462)
Ending balance, shares at Jun. 30, 2019		69,913,410
Issuance of common stock upon exercise of stock options, value	25		25
Issuance of common stock upon exercise of stock options, shares		52,000
Issuance of common stock under ESPP plan, value	89		89
Issuance of common stock under ESPP plan, shares		28,542
Stock-based compensation expense	2,037		2,037
Unrealized gain on investments	7				7
Net loss	(16,409)			(16,409)
Ending balance at Sep. 30, 2019	$ (32,219)	$ 70	$ 408,575	$ (440,871)	$ 7
Ending balance, shares at Sep. 30, 2019	69,993,952	69,993,952