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Income Taxes - Reconciliation of Income Taxes Computed at Federal Statutory Rate and Provision for Income Taxes (Detail)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]      
Expected income tax 21.00% 34.00% 34.00%
State taxes, net of federal benefit (0.20%) (0.20%) (0.10%)
Permanent differences 0.20% (0.40%) (0.40%)
Stock compensation 9.30% 4.60% (1.00%)
Federal research and development credit 1.20% 1.00% 2.40%
Foreign rate differential (1.10%) (8.70%) (3.00%)
Change in valuation allowance (32.80%) (26.10%) (31.90%)
Other 1.50% 0.50% 0.10%
Total income tax expense (benefit) (0.90%) 4.70% 0.10%