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Summary of Significant Accounting Policies - Difference Between Federal Statutory Rate and Effective Tax Rate (Detail)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Income Tax Disclosure [Abstract]        
Tax at statutory rate 21.00% 35.00% 21.00% 35.00%
Tax effect of:        
Valuation allowance on deferred tax assets (24.10%) (42.10%) (24.30%) (40.10%)
State income taxes, net of federal benefit 3.90% 5.00% 3.50% 4.30%
Nondeductible stock-based compensation (0.40%) (0.40%) (0.70%) (1.40%)
Effective tax rate 0.40% (2.50%) (0.50%) (2.20%)