Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses:
Research and development	$ 16,297,617	$ 14,908,843	$ 6,947,439
General and administrative	7,833,085	7,379,339	4,650,807
Total operating expenses	24,130,702	22,288,182	11,598,246
Other income (expense):
Interest income	67,885	57,616	22,144
Interest expense	(70,000)	(70,000)	(462,570)
Other expense	(15,809)	(19,325)	(1,070)
Other financing expense			(3,576,750)
Total other expense	(17,924)	(31,709)	(4,018,246)
Net loss	(24,148,626)	(22,319,891)	(15,616,492)
Other comprehensive income (loss):
Net unrealized gains (losses) on marketable securities	9,390	(24,794)	10,946
Comprehensive loss	(24,139,236)	(22,344,685)	(15,605,546)
Reconciliation of net loss to net loss applicable to common stockholders:
Net loss	(24,148,626)	(22,319,891)	(15,616,492)
Gain on extinguishment of convertible preferred stock			11,491,043
Deemed distribution from promissory note issuance			(474,561)
Net loss applicable to common stockholders	$ (24,148,626)	$ (22,319,891)	$ (4,600,010)
Net loss per share applicable to common stockholders, basic and diluted	$ (1.30)	$ (1.44)	$ (0.63)
Weighted average shares outstanding used in computing net loss per share applicable to common stockholders, basic and diluted	18,617,537	15,478,999	7,358,201