Condensed Statements of Cash Flows - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities
Net loss	$ (3,622,745)	$ (7,271,884)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	26,089	26,725
Stock-based compensation expense	1,249,473	901,924
Amortization of premium on marketable securities	7,335	22,122
Accrued interest included in convertible note payable	92,466
Changes in operating assets and liabilities:
Other receivables	2,500,000
Prepaid and other current assets	(8,691)	224,354
Other assets	(203,094)
Accounts payable and accrued expenses	367,652	(252,818)
Accrued compensation	(1,412,968)	(544,584)
Deferred revenue	(6,998,076)
Deferred rent	(6,924)	(3,850)
Net cash used in operating activities	(8,009,483)	(6,898,011)
Investing activities
Maturities of marketable securities	8,994,000	13,875,000
Purchase of marketable securities	(54,148,513)	(4,240,142)
Capital expenditures	(2,418)	(105,114)
Net cash (used in) provided by investing activities	(45,156,931)	9,529,744
Financing activities
Proceeds from issuance of convertible promissory note, net	12,500,000
Principal payment on promissory note	(1,000,000)
Proceeds from issuance of common stock, net		3,076,709
Proceeds from stock issuances under employee stock purchase plan and exercise of stock options	58,487	1,503
Net cash provided by financing activities	11,558,487	3,078,212
Net (decrease) increase in cash and cash equivalents	(41,607,927)	5,709,945
Cash and cash equivalents at beginning of period	58,083,409	13,876,090
Cash and cash equivalents at end of period	16,475,482	19,586,035
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 4,861	$ 17,500