Stock-Based Compensation - Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rates, Minimum	1.97%	0.74%	1.49%
Risk-free interest rates, Maximum	2.29%	1.75%	1.92%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	6 years	5 years 7 months 6 days	6 years
Expected volatility, Minimum	69.90%	67.30%	76.40%
Expected volatility, Maximum	72.80%	99.90%	83.90%