Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2015	$ 205,394	$ 39	$ 287,937	$ 30	$ (82,612)
Beginning balance, shares at Dec. 31, 2015		39,082,017
Issuance of common stock upon exercise of stock options, value	2,138	$ 1	2,137
Issuance of common stock upon exercise of stock options, shares		1,273,736
Stock-based compensation expense	16,857		16,857
Unrealized gain (loss) on investments	(179)			(179)
Net loss	(91,579)				(91,579)
Ending balance at Dec. 31, 2016	132,631	$ 40	306,931	(149)	(174,191)
Ending balance, shares at Dec. 31, 2016		40,355,753
Issuance of common stock upon exercise of stock options, value	116		116
Issuance of common stock upon exercise of stock options, shares		174,386
Issuance of common stock upon vesting of RSUs, net of tax withholdings	(33)		(33)
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		40,876
Stock-based compensation expense	17,362		17,362
Unrealized gain (loss) on investments	3			3
Net loss	(89,380)				(89,380)
Ending balance at Dec. 31, 2017	$ 60,699	$ 40	324,376	(146)	(263,571)
Ending balance, shares at Dec. 31, 2017	40,571,015	40,571,015
Issuance of common stock upon exercise of stock options, value	$ 147	$ 1	146
Issuance of common stock upon exercise of stock options, shares	212,240	212,240
Issuance of common stock upon vesting of RSUs, net of tax withholdings	$ 61		61
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		138,048
Repurchase of common stock for employee tax withholdings	(197)		(197)
Repurchase of common stock for employee tax withholdings, shares		(17,900)
Issuance of common stock under ESPP plan, value	257		257
Issuance of common stock under ESPP plan, shares		33,332
Stock-based compensation expense	16,641		16,641
Unrealized gain (loss) on investments	146			146
Adoption of new revenue standard (ASC 606)	(26,857)				(26,857)
Net loss	(98,942)				(98,942)
Ending balance at Dec. 31, 2018	$ (48,045)	$ 41	$ 341,284	$ 0	$ (389,370)
Ending balance, shares at Dec. 31, 2018	40,936,735	40,936,735