Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (24,148,626)	$ (22,319,891)	$ (15,616,492)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	67,856	38,258	10,446
Stock-based compensation expense	3,315,943	2,330,660	257,451
Noncash other financing expense			3,618,206
Amortization of premium on marketable securities	328,479	577,158	199,182
Changes in operating assets and liabilities:
Prepaid and other current assets	(1,185,213)	(251,314)	(469,320)
Other assets	(798,754)	(62,981)
Accounts payable and accrued expenses	(581,218)	1,499,601	407,089
Accrued compensation	289,587	(130,345)	961,721
Deferred rent	165,761	58,699
Net cash used in operating activities	(22,546,185)	(18,260,155)	(10,631,717)
Investing activities
Maturities of marketable securities	62,574,000	54,072,000	4,725,000
Purchase of marketable securities	(58,365,836)	(29,639,190)	(53,117,797)
Capital expenditures	(88,524)	(242,681)	(3,910)
Net cash provided by (used in) investing activities	4,119,640	24,190,129	(48,396,707)
Financing activities
Issuance of promissory notes			1,001,439
Distribution to wholly owned subsidiary in connection with spin-off of Idun Pharmaceuticals, Inc.			(500,000)
Deferred public offering costs		(264,675)
Proceeds from issuance of common stock, net of offering costs	22,305,226		58,608,454
Proceeds from stock issuances under employee stock purchase plan and exercise of stock options	84,735	88,422	41,393
Net cash provided by (used in) financing activities	22,389,961	(176,253)	59,151,286
Net increase in cash and cash equivalents	3,963,416	5,753,721	122,862
Cash and cash equivalents at beginning of period	9,912,674	4,158,953	4,036,091
Cash and cash equivalents at end of period	13,876,090	9,912,674	4,158,953
Supplemental disclosure of cash flow information:
Cash paid for interest	70,000	$ 70,000	88,583
Supplemental schedule of noncash investing and financing activities:
Purchases of property and equipment included in accounts payable	$ 87,000
Conversion of notes payable for common stock			1,001,439
Issuance of warrants in conjunction with debt			$ 625,792