Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES
Consolidated net loss	$ (118,476,012)	$ (78,252,894)	$ (62,948,646)
Adjustments to reconcile consolidated net loss to net cash used in operating activities:
Gain on sale of long-term securities	0	(33,042)	0
Noncash stock compensation expense	15,945,284	7,509,999	15,697,092
Depreciation	82,355	62,960	15,452
Amortization of premium on investment securities	61,320	459,429	536,142
Change in fair value of notes payable and accrued interest	58,739	(109,657)	(56,717)
Changes in assets and liabilities:
Increase in restricted cash	(4,083)	(4,065)	(4,131)
Decrease (increase) in other current assets	(2,665,222)	3,564,316	(3,105,771)
Decrease (increase) in leasehold interest	124,947	(2,042,281)	0
(Increase) decrease in accrued interest receivable	(24,616)	102,169	(100,505)
Decrease (increase) in other assets	161,730	(4,784)	(41,722)
Increase (decrease) in accounts payable and accrued expenses	11,020,034	6,492,644	5,470,915
Increase in deferred rent	104,343	816,257	0
Decrease in deferred revenue	(152,381)	(152,381)	(152,381)
Net cash used in operating activities	(93,763,562)	(61,591,330)	(44,690,272)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of equipment	(2,227)	(343,985)	(42,217)
Investment in held-to-maturity securities	(28,006,270)	(15,199,922)	(48,993,652)
Proceeds from maturity of short-term securities	19,800,000	29,500,000	24,350,000
Proceeds from the sale of long-term securities	0	12,589,219	0
Net cash provided by (used in) investing activities	(8,208,497)	26,545,312	(24,685,869)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from the exercise of warrants	2,143,085	618,242	1,067,339
Proceeds from sale of common stock, net	131,515,541	4,411,233	67,760,517
Deferred financing costs paid	0	(13,506)	(104,170)
Net cash provided by financing activities	133,658,626	5,015,969	68,723,686
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	31,686,567	(30,030,049)	(652,455)
Cash and cash equivalents at beginning of year	25,031,280	55,061,329	55,713,784
CASH AND CASH EQUIVALENTS AT END OF YEAR	56,717,847	25,031,280	55,061,329
NONCASH TRANSACTIONS
Reclassification of deferred financing costs to additional paid-in capital	(2,645)	(25,097)	(111,810)
Conversion of convertible notes payable to common stock	$ 0	$ 33,017	$ 6,924