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, 2012	$ 62,093		$ 36		$ 122,528		$ (52)	$ (60,419)
Beginning balance (in shares) at Dec. 31, 2012			35,831,808
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)			732,483
Issuance of common stock upon exercise of options	594		$ 1		593
Stock-based compensation expense	3,422				3,422
Issuance of common stock under Employee Stock Purchase Plan (in shares)			48,035
Issuance of common stock under Employee Stock Purchase Plan	201				201
Issuance of common stock (in shares)			5,175,000
Issuance of common stock	45,779		$ 5		45,774
Unrealized gain (loss) on short-term investments	36						36
Net loss	(18,668)							(18,668)
Ending balance at Dec. 31, 2013	93,457		$ 42		172,518		(16)	(79,087)
Ending 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]
Common stock, shares outstanding	48,944,530
Issuance of common stock upon exercise of options (in shares)	2,299,000		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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, shares outstanding	52,669,266		52,669,266