Annual report pursuant to Section 13 and 15(d)

Provision for Income Taxes - Summary of Valuation Allowance (Detail)

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Provision for Income Taxes - Summary of Valuation Allowance (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Valuation Allowance [Abstract]    
Valuation allowance, at beginning of year $ 18,278 $ 9,852
Change in valuation allowance 16,955 8,426
Valuation allowance, at end of year $ 35,233 $ 18,278