Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating loss carryforwards	$ 76,741,198	$ 34,870,169
Accrued employee benefits	543,466	443,371
Research and development credit	6,962,731	2,570,540
Stock compensation	5,641,993	1,563,785
Deferred rent	725,195
Deferred tax liabilities:
Depreciation	(6,062)	(5,204)
Net deferred tax asset	90,608,521	39,442,661
Valuation allowance	(90,608,521)	(39,442,661)
Net deferred tax asset	$ 0	$ 0