Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($)	Total	Series A Warrants [Member]	Series B Warrants [Member]	Preferred Stock [Member] Series A Preferred Stock [Member]	Preferred Stock [Member] Series B Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] Series A Warrants [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Series A Warrants [Member]	Additional Paid-in Capital [Member] Series B Warrants [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2012	$ (58,335,871)			$ 32,208,532	$ 31,699,840	$ 105		$ 470,982			$ 551	$ (58,807,509)
Beginning balance, shares at Dec. 31, 2012				42,494,218	36,417,224	1,052,606
Vesting of early exercise of employee stock options	5,826					$ 9		5,817
Vesting of early exercise of employee stock options, shares						92,561
Issuance of common stock upon exercise of stock options	$ 273					$ 1		272
Issuance of common stock upon exercise of stock options, shares	174,447					3,030
Share-based compensation	$ 257,451							257,451
Investment in Idun Pharmaceuticals, Inc.	(500,000)							(493,507)				(6,493)
Deemed distribution from bridge note issuance	(474,561)							(474,561)
Issuance of common stock, net of offering costs	58,608,454					$ 600		58,607,854
Issuance of common stock, net of offering costs, shares						6,000,000
Conversion of warrants to common stock		$ 3,083,549	$ 1,159,659				$ 28		$ 3,083,521	$ 1,159,659
Conversion of warrants to common stock, shares							280,675
Conversion of preferred stock to common stock	63,908,235			$ (32,208,532)	$ (31,699,840)	$ 787		63,907,448
Conversion of preferred stock to common stock, shares				(42,494,218)	(36,417,224)	7,865,722
Conversion of notes payable and interest to common stock	1,011,481					$ 9		1,011,472
Conversion of notes payable and interest to common stock, shares						91,948
Net loss	(15,616,492)											(15,616,492)
Unrealized gain (loss) on marketable securities	10,946										10,946
Ending balance at Dec. 31, 2013	53,118,950					$ 1,539		127,536,408			11,497	(74,430,494)
Ending balance, shares at Dec. 31, 2013						15,386,542
Vesting of early exercise of employee stock options	20,602					$ 10		20,592
Vesting of early exercise of employee stock options, shares						93,016
Issuance of common stock upon exercise of stock options	$ 88,422					$ 7		88,415
Issuance of common stock upon exercise of stock options, shares	81,056					81,056
Share-based compensation	$ 2,330,660							2,330,660
Net loss	(22,319,891)											(22,319,891)
Unrealized gain (loss) on marketable securities	(24,794)										(24,794)
Ending balance at Dec. 31, 2014	33,213,949					$ 1,556		129,976,075			(13,297)	(96,750,385)
Ending balance, shares at Dec. 31, 2014						15,560,614
Vesting of early exercise of employee stock options	24,404					$ 9		24,395
Vesting of early exercise of employee stock options, shares						71,249
Issuance of common stock upon exercise of stock options	$ 19,212							19,212
Issuance of common stock upon exercise of stock options, shares	21,029					21,029
Issuance of common stock for employee stock purchase plan	$ 65,523					$ 2		65,521
Issuance of common stock for employee stock purchase plan, shares						17,914
Share-based compensation	3,315,943							3,315,943
Issuance of common stock, net of offering costs	22,040,551					$ 417		22,040,134
Issuance of common stock, net of offering costs, shares						4,174,805
Net loss	(24,148,626)											(24,148,626)
Unrealized gain (loss) on marketable securities	9,390										9,390
Ending balance at Dec. 31, 2015	$ 34,540,346					$ 1,984		$ 155,441,280			$ (3,907)	$ (120,899,011)
Ending balance, shares at Dec. 31, 2015						19,845,611