Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Accumulated deficit
Beginning balance (in shares) at Dec. 31, 2014		48,944,530
Beginning balance at Dec. 31, 2014	$ 132,014	$ 49	$ 267,929	$ (197)	$ (135,767)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		2,298,618
Issuance of common stock upon exercise of options	6,680	$ 2	6,678
Stock-based compensation expense	15,368		15,368
Issuance of common stock under Employee Stock Purchase Plan (in shares)		69,380
Issuance of common stock under Employee Stock Purchase Plan	492		492
Conversion of convertible note payable into common stock (in shares)		1,356,738
Conversion of convertible note payable into common stock	25,208	$ 2	25,206
Unrealized gain (loss) on short-term investments	64			64
Net loss	(55,748)				(55,748)
Ending balance (in shares) at Dec. 31, 2015		52,669,266
Ending balance at Dec. 31, 2015	124,078	$ 53	315,673	(133)	(191,515)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		89,855
Issuance of common stock upon exercise of options	310		310
Stock-based compensation expense	12,872		12,872
Issuance of common stock under Employee Stock Purchase Plan (in shares)		165,684
Issuance of common stock under Employee Stock Purchase Plan	641		641
Unrealized gain (loss) on short-term investments	10			10
Net loss	(81,836)				(81,836)
Ending balance (in shares) at Dec. 31, 2016		52,924,805
Ending balance at Dec. 31, 2016	$ 56,075	$ 53	329,496	(123)	(273,351)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)	11,000	11,000
Issuance of common stock upon exercise of options	$ 4	$ 0	4
Stock-based compensation expense	7,642		7,642
Issuance of common stock under Employee Stock Purchase Plan (in shares)		419,342
Issuance of common stock under Employee Stock Purchase Plan	419		419
Unrealized gain (loss) on short-term investments	(11)			(11)
Issuance of common stock, net of offering costs (in shares)		50,600,000
Issuance of common stock, net of offering costs	42,992	$ 51	42,941
Cumulative effect of accounting change	0		602		(602)
Net loss	(71,905)				(71,905)
Ending balance (in shares) at Dec. 31, 2017		103,955,147
Ending balance at Dec. 31, 2017	$ 35,216	$ 104	$ 381,104	$ (134)	$ (345,858)