Statements of Changes in Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Hercules Notes Additional Paid-in Capital	Hercules Notes	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common stock	Additional Paid-in Capital	Accumulated Deficit	Total
Balance at beginning of the period at Dec. 31, 2012						$ 2,745	$ (23,932)	$ (21,187)
Balance at beginning of the period at Dec. 31, 2012			22,844
Balance at beginning of the period (in shares) at Dec. 31, 2012			26,750,540
Balance at beginning of the period (in shares) at Dec. 31, 2012					210,586
Increase (Decrease) in Convertible Preferred Stock
Exercise of preferred stock warrant			504
Exercise of preferred stock warrant (in shares)			498,306
Exercise of common stock warrant						2		2
Exercise of common stock warrant ( in Shares)					2,836
Exercise of stock options						21		21
Exercise of stock options (in shares)					23,604
Beneficial conversion feature						2,440		2,440
Stock-based compensation expense and vesting of restricted stock						140		140
Net loss							(20,795)	(20,795)
Balance at end of the period at Dec. 31, 2013						5,348	(44,727)	(39,379)
Balance at end of the period at Dec. 31, 2013			23,348
Balance at end of the period (in shares) at Dec. 31, 2013			27,248,746
Balance at end of the period (in shares) at Dec. 31, 2013					237,026			237,026
Increase (Decrease) in Convertible Preferred Stock
Exercise of preferred stock warrant			6,436	993
Exercise of preferred stock warrant (in shares)			5,101,816	787,402
Exercise of stock options						90		90
Exercise of stock options (in shares)					98,405
Issuance of preferred stock			13,184	30,478
Issuance of preferred stock ( in shares)			13,184,383	24,409,444
Issuance of convertible notes						2,071		2,071
Extinguishment of convertible notes				5,013		(1,150)		(1,150)
Extinguishment of convertible notes (in shares)				3,947,363
Exchange of preferred stock for common stock			(42,968)	(36,484)	10	79,442		79,452
Exchange of preferred stock for common stock ( in shares)			(45,534,945)	(29,144,209)	9,901,700
Issuance of common stock in connection with the Merger					3	31,321		31,324
Issuance of common stock in connection with the Merger ( in shares)					2,610,871
Exchange of warrants in connection with the Merger	248	248				167		167
Issuance of common stock upon vesting of restricted stock units						(297)		(297)
Issuance of common stock upon vesting of restricted stock units ( in shares)					27,540
Stock-based compensation expense and vesting of restricted stock						1,719		1,719
Stock-based compensation expense and vesting of restricted stock ( in shares)					45,301
Net loss							(41,844)	(41,844)
Balance at end of the period at Dec. 31, 2014					$ 13	$ 118,959	$ (86,571)	$ 32,401
Balance at end of the period (in shares) at Dec. 31, 2014			0
Balance at end of the period (in shares) at Dec. 31, 2014					12,920,843			920,843