Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series C Convertible Preferred Stock	Series D Convertible Preferred Stock	Other Comprehensive Loss	Founder	Nonemployee	Common Stock	Common Stock Founder	Common Stock Nonemployee	Additional Paid-In Capital	Additional Paid-In Capital Founder	Additional Paid-In Capital Nonemployee	Accumulated Deficit	Initial Public Offering	Initial Public Offering Convertible Preferred Stock	Initial Public Offering Common Stock	Initial Public Offering Additional Paid-In Capital
Balance at Dec. 31, 2012	$ (8,571)	$ 10,722									$ 9,194			$ (17,765)
Balance, shares at Dec. 31, 2012		1,704,000						58,000
Issuance of stock			$ 8,676	$ 34,884
Issuance of stock, shares			1,823,000	4,162,000
Vesting of common stock issued, shares									3,000
Exercise of stock options	$ 65										65
Exercise of stock options, shares	100,273							100,000
Stock-based compensation expense	$ 70										70
Net loss	(12,377)													(12,377)
Balance at Dec. 31, 2013	(20,813)	$ 54,282									9,329			(30,142)
Balance, shares at Dec. 31, 2013		7,689,000						161,000
Issuance of stock	71,624			$ 15,959				$ 1			71,623
Issuance of stock, shares				1,903,000				8,000,000
Vesting of common stock issued						$ 1	$ 54					$ 1	$ 54
Vesting of common stock issued, shares									1,000	21,000
Exercise of stock options	$ 282										282
Exercise of stock options, shares	168,523							168,000
Stock-based compensation expense	$ 689										689
Net loss	(21,712)													(21,712)
Balance at Dec. 31, 2014	100,366							$ 2			152,218			(51,854)
Balance, shares at Dec. 31, 2014								17,943,000
Conversion of preferred stock to common stock															$ 70,241	$ (70,241)	$ 1	$ 70,240
Conversion of preferred stock to common, shares																(9,592,000)	9,592,000
Exercise of stock options	$ 365										365
Exercise of stock options, shares	218,636							219,000
Stock-based compensation expense	$ 3,272										3,272
Net loss	(32,644)													(32,644)
Unrealized loss on available-for-sale securities	(69)				$ (69)
Balance at Dec. 31, 2015	71,788				$ (69)			$ 2			156,353			$ (84,498)
Balance, shares at Dec. 31, 2015								18,232,000
Issuance of common stock per employee stock purchase plan	$ 498										$ 498
Issuance of common stock per employee stock purchase plan, shares								70,000