CONDENSED STATEMENTS OF CASH FLOWS - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net loss	$ (32,774,814)	$ (46,500,131)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	597,557	701,133
Stock-based compensation	4,451,096	2,680,873
Acquired in-process research and development		16,493,132
Loss (gain) on disposal of property and equipment	(6,029)	114,042
Amortization of debt discount	(1,282,175)
Accretion of bond premium		1,399
Change in operating assets and liabilities:
Receivables	224,880	157,805
Prepaid expenses and other assets	(8,398,316)	770,892
Accounts payable	557,027	(1,160,801)
Accrued wages, benefits and other liabilities	2,391,338	(633,288)
Deferred revenue	(47,565)	(37,500)
Net cash used in operating activities	(34,287,001)	(27,412,444)
Investing activities
Purchases of property and equipment	(43,094)	(46,833)
Proceeds from disposal of property and equipment	90,161
Purchases of investments	(308,005,868)	(51,318,003)
Purchase of acquired in-process research and development		(12,322,349)
Proceeds from sale and maturities of investments	111,300,000	102,430,000
Net cash provided by (used in) investing activities	(196,658,801)	38,742,815
Financing activities
Stock repurchase	(104,461)	(94,627)
Proceeds from exercise of warrant to purchase common stock		4,556,420
Proceeds from issuance of common stock in connection with equity offering	269,825,618
Proceeds from the exercise of stock options	9,838,785	109,742
Proceeds from stock purchases under employee stock purchase plan	95,070	48,407
Net cash provided by financing activities	279,655,012	4,619,942
Net increase in cash and cash equivalents	48,709,210	15,950,313
Cash and cash equivalents at beginning of period	18,559,130	31,228,192
Cash and cash equivalents at end of period	$ 67,268,340	$ 47,178,505