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Income Taxes - Schedule of Reconciliation Between U.S. Statutory Income Tax Rate and Company's Effective Rate (Detail)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]      
U.S. federal income tax statutory rate 21.00% 35.00% 35.00%
Permanent differences   (3.00%) (4.00%)
State tax, net of federal benefit 2.00% (5.00%) (1.00%)
Changes in valuation allowance for deferred tax assets (22.00%) 113.00% (30.00%)
Stock-based compensation     (1.00%)
2017 Tax Cuts and Jobs Act   (138.00%)  
Other (1.00%) (2.00%) 1.00%
Effective tax rate 0.00% 0.00% 0.00%