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Summary of Significant Accounting Policies - Difference Between Federal Statutory Rate and Effective Tax Rate on the Income (Loss) Before Income Taxes Due (Detail)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Income Tax Disclosure [Abstract]    
Tax at statutory rate 21.00% 35.00%
Tax effect of:    
Valuation allowance on deferred tax assets (24.50%) (38.70%)
State income taxes, net of federal benefit 3.10% 4.20%
Nondeductible stock-based compensation (1.00%) (2.40%)
Effective tax rate (1.40%) (1.90%)