Income Taxes - Reconciliation of Federal Statutory Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Federal statutory income (benefit) tax rate	21.00%	(34.00%)	34.00%
2017 Tax Act, net deferred tax remeasurement		(626.73%)
State taxes, net of federal benefit	(0.11%)	(2.01%)	2.88%
Stock compensation	(4.74%)	(5.18%)	(0.61%)
Permanent differences-other	(1.33%)	(13.39%)	(1.00%)
Research and development ("R&D") credit		5.54%	0.98%
Valuation release for bargain purchase gain		(302.23%)
Other	(2.07%)	(1.36%)	(0.47%)
Decrease (increase) in valuation allowance	(12.65%)	709.88%	(35.78%)
Federal statutory income tax rate, Total	0.10%	(302.23%)	0.00%
North Carolina [Member]
Income Taxes [Line Items]
North Carolina tax rate change		(32.75%)