Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Private Placement	Common stock	Common stock Private Placement	Additional paid-in capital	Additional paid-in capital Private Placement	Accumulated other comprehensive income (loss)	Accumulated deficit
Beginning balance at Dec. 31, 2013	$ 93,457		$ 42		$ 172,518		$ (16)	$ (79,087)
Beginning balance (in shares) at Dec. 31, 2013			41,787,326
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)			1,006,515
Issuance of common stock upon exercise of options	2,233		$ 1		2,232
Stock-based compensation expense	7,039				7,039
Issuance of common stock under Employee Stock Purchase Plan (in shares)			38,085
Issuance of common stock under Employee Stock Purchase Plan	283				283
Issuance of common stock (in shares)			4,808,824	1,303,780
Issuance of common stock	76,294	$ 9,569	$ 5	$ 1	76,289	$ 9,568
Unrealized gain (loss) on short-term investments	(181)						(181)
Net loss	(56,680)							(56,680)
Ending balance at Dec. 31, 2014	132,014		$ 49		267,929		(197)	(135,767)
Ending balance (in shares) at Dec. 31, 2014			48,944,530
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 at Dec. 31, 2015	$ 124,078		$ 53		315,673		(133)	(191,515)
Ending balance (in shares) at Dec. 31, 2015			52,669,266
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)	90,000		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 at Dec. 31, 2016	$ 56,075		$ 53		$ 329,496		$ (123)	$ (273,351)
Ending balance (in shares) at Dec. 31, 2016			52,924,805