Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Treasury Stock	Unrealized Gain/Loss	Accumulated Deficit	IPO	IPO Additional Paid-In Capital	Series A redeemable convertible preferred stock	Series B redeemable convertible preferred stock	Series C redeemable convertible preferred stock	2012 Series X Redeemable Convertible Preferred Stock
Balance at Dec. 31, 2012	$ (59,588)					$ (59,588)
Balance, shares at Dec. 31, 2012		615,757
Balance at Dec. 31, 2012									$ 37,092	$ 19,816
Balance, shares at Dec. 31, 2012									734,538	1,287,525
Issuance of restricted common stock, shares		583,126
Issuance of common stock, net of issuance costs, shares		176,717
Exercise of stock options, shares		14,357
Exercise of stock options	5		$ 5
Forfeitures of restricted common stock, shares		(6,612)
Reclassification of 2012 Series X preferred stock upon modification												$ 2,486
Reclassification of 2012 Series X preferred stock upon modification, shares												25,000
Sale of preferred stock, net of issuance cost											$ 40,088	$ 2,458
Sale of preferred stock, net of issuance cost, shares											2,945,742	25,000
Conversion of preferred stock, shares											357,143	(50,000)
Conversion of preferred stock											$ 4,944	$ (4,944)
Accretion of preferred stock to redemption value									$ 2,275	$ 1,441	52,170
Accretion of preferred stock to redemption value	(55,886)		(1,569)			(54,317)
Share-based compensation expense	1,564		1,564
Net income (loss)	(13,167)					(13,167)
Balance at Dec. 31, 2013	(127,072)					(127,072)
Balance, shares at Dec. 31, 2013		1,383,345
Balance at Dec. 31, 2013									$ 39,367	$ 21,257	$ 97,203
Balance, shares at Dec. 31, 2013									734,538	1,287,525	3,302,885
Issuance of restricted common stock, shares		56,000
Issuance of common stock, net of issuance costs	114,954		114,954
Issuance of common stock, net of issuance costs, shares		6,762,000
Receipt of payment on promissory notes issued in exchange for shares of common stock	237		237
Exercise of stock options, shares		116,394
Exercise of stock options	89		89
Forfeitures of restricted common stock, shares		(53,835)
Conversion of preferred stock, shares		12,115,183							(734,538)	(1,287,525)	(3,302,885)
Conversion of preferred stock	244,727		180,057			64,670			$ (74,188)	$ (45,514)	$ (125,025)
Accretion of preferred stock to redemption value									$ 34,821	$ 24,257	$ 27,822
Accretion of preferred stock to redemption value	(86,900)		(85,663)			(1,237)
Share-based compensation expense	6,010		6,010
Deferred offering costs- IPO							$ (10,715)	$ (10,715)
Unrealized gain/loss	(56)				$ (56)
Treasury shares purchased, not retired( 2,553)	(94)			$ (94)
Treasury shares purchased, not retired in shares		(2,553)
Treasury shares purchased, retired (5,910)	(68)			(68)
Treasury shares purchased, retired in shares		(5,910)
Net income (loss)	(37,034)					(37,034)
Balance at Dec. 31, 2014	$ 104,078		204,969	(162)	(56)	(100,673)
Balance, shares at Dec. 31, 2014	20,370,624	20,370,624
Issuance of common stock, net of issuance costs	$ 82,750		82,750
Issuance of common stock, net of issuance costs, shares		10,083,070
Exercise of stock options, shares	218,674	218,674
Exercise of stock options	$ 130		130
Proceeds from sale of stock under employee stock purchase plan	220		220
Proceeds from sale of stock under employee stock purchase plan, shares		25,903
Forfeitures of restricted common stock, shares		(36,053)
Share-based compensation expense	4,714		4,714
Unrealized gain/loss	(178)				(178)
Net income (loss)	(60,716)					(60,716)
Balance at Dec. 31, 2015	$ 130,998		$ 292,783	$ (162)	$ (234)	$ (161,389)
Balance, shares at Dec. 31, 2015	30,662,218	30,662,218