CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($)	Common Stock	Additional Paid in Capital	Accumulated Deficit	Total
Beginning balance at Dec. 31, 2015	$ 177,928	$ 282,712,078	$ (219,826,860)	$ 63,063,146
Beginning balance (in shares) at Dec. 31, 2015	177,928,041
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued in offerings, net of cost	$ 17,424	134,846,051		134,863,475
Shares issued in offerings, net of cost (in shares)	17,424,242
Shares issued for exercise of options, net	$ 723	1,372,277		1,373,000
Shares issued for exercise of options, net (in shares)	722,744
Shares issued for exercise of warrants, net	$ 613	988,447		989,060
Shares issued for exercise of warrants, net (in shares)	613,195
Share-based compensation		17,076,199		17,076,199
Net loss			(89,875,459)	(89,875,459)
Ending balance at Dec. 31, 2016	$ 196,688	436,995,052	(309,702,319)	127,489,421
Ending balance (in shares) at Dec. 31, 2016	196,688,222
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued in offerings, net of cost	$ 12,400	68,560,235		68,572,635
Shares issued in offerings, net of cost (in shares)	12,400,000
Shares issued for exercise of options, net	$ 103	212,512		212,615
Shares issued for exercise of options, net (in shares)	102,546
Shares issued for exercise of warrants, net	$ 7,239	3,791,760		3,798,999
Shares issued for exercise of warrants, net (in shares)	7,238,874
Share-based compensation		6,760,425		6,760,425
Adoption of ASU 2016-09 at Dec. 31, 2016		31,421	(31,421)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss			(76,925,380)	(76,925,380)
Ending balance at Dec. 31, 2017	$ 216,430	516,351,405	(386,659,120)	$ 129,908,715
Ending balance (in shares) at Dec. 31, 2017	216,429,642			216,429,642
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued in offerings, net of cost	$ 18,578	89,889,219		$ 89,907,797
Shares issued in offerings, net of cost (in shares)	18,578,430
Shares issued for exercise of options, net	$ 5,455	1,660,753		1,666,208
Shares issued for exercise of options, net (in shares)	5,454,367
Share-based compensation		8,658,561		8,658,561
Net loss			(132,617,160)	(132,617,160)
Ending balance at Dec. 31, 2018	$ 240,463	$ 616,559,938	$ (519,276,280)	$ 97,524,121
Ending balance (in shares) at Dec. 31, 2018	240,462,439			240,462,439