Stock-Based Compensation - Schedule of Estimated Assumptions Used to Calculate Weighted-Average Grant Date Fair Value of Options Granted (Detail) - Stock Options [Member] - 2006 and 2014 Stock Option Plans and Inducement Plan [Member]	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-Based Compensation
Weighted-average risk-free interest rate	2.52%	2.63%
Expected dividend yield	0.00%	0.00%
Expected volatility	71.00%	68.00%
Expected terms	6 years	6 years