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Income Taxes - Schedule of Valuation Allowance - (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Income Tax Disclosure [Abstract]    
Balance at beginning of year $ (26,515) $ (13,060)
Increase in valuation allowance (13,775) (13,455)
Balance at end of year $ (40,290) $ (26,515)