Condensed Statements of Cash Flows - USD ($)	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating activities
Net loss	$ (9,039,632)	$ (13,743,029)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	53,723	53,518
Stock-based compensation expense	2,062,924	1,710,423
Amortization of premium on marketable securities	9,595	27,475
Accrued interest included in convertible note payable	279,452	0
Changes in operating assets and liabilities:
Other receivables	(4,500,000)	0
Prepaid and other current assets	(136,433)	1,105,208
Other assets	(872,111)	0
Accounts payable and accrued expenses	3,262,191	(453,147)
Accrued compensation	(985,135)	(195,343)
Deferred revenue	(10,006,507)	0
Deferred rent	(13,847)	(7,700)
Net cash used in operating activities	(19,885,780)	(11,502,595)
Investing activities
Maturities of marketable securities	28,309,000	24,347,000
Purchase of marketable securities	(81,836,441)	(18,462,944)
Capital expenditures	(20,682)	(107,165)
Net cash (used in) provided by investing activities	(53,548,123)	5,776,891
Financing activities
Proceeds from issuance of convertible promissory note, net	12,500,000	0
Principal payment on promissory note	(1,000,000)	0
Proceeds from issuance of common stock, net	30,740,665	5,275,872
Repurchase of common stock	(11,202,542)	0
Proceeds from stock issuances under employee stock purchase plan and exercise of stock options	77,640	32,898
Net cash provided by financing activities	31,115,763	5,308,770
Net decrease in cash and cash equivalents	(42,318,140)	(416,934)
Cash and cash equivalents at beginning of period	58,083,409	13,876,090
Cash and cash equivalents at end of period	15,765,269	13,459,156
Supplemental disclosure of cash flow information:
Cash paid for interest	4,861	35,000
Supplemental schedule of noncash financing activities:
Costs related to issuance of common stock included in accounts payable and accrued expenses	$ 131,122	$ 0