CONDENSED STATEMENTS OF CASH FLOWS - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities
Net loss	$ (20,167,289)	$ (23,233,313)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	401,670	468,994
Stock-based compensation	2,800,485	1,960,172
Loss (gain) on disposal of property and equipment	(18,177)	144,578
Amortization of debt discount	(596,273)
Accretion of bond premium		46,770
Change in operating assets and liabilities:
Receivables	145,956	173,375
Prepaid expenses and other assets	(2,050,330)	1,034,660
Accounts payable	422,702	(182,338)
Accrued wages, benefits and other liabilities	334,078	(179,369)
Deferred revenue	(31,709)	(25,000)
Net cash used in operating activities	(18,758,887)	(19,791,471)
Investing activities
Purchases of property and equipment	(14,708)	(43,838)
Proceeds from disposal of property and equipment	89,144
Purchases of investments	111,009,044	20,457,875
Proceeds from sale and maturities of investments	48,800,000	73,930,000
Net cash provided by investing activities	(62,134,608)	53,428,287
Financing activities
Stock repurchase	(104,461)	(94,627)
Proceeds from issuance of common stock in connection with equity offering	80,900,581
Proceeds from the exercise of stock options	6,488,131	109,742
Proceeds from stock purchases under employee stock purchase plan	95,070	48,407
Net cash provided by financing activities	87,379,321	63,522
Net increase in cash and cash equivalents	6,485,826	33,700,338
Cash and cash equivalents at beginning of period	18,559,130	31,228,192
Cash and cash equivalents at end of period	$ 25,044,956	$ 64,928,530