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Income Taxes - Summary of Changes to Valuation Allowance Balance (Detail) - Valuation Allowance of Deferred Tax Assets [Member] - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Valuation Allowance [Line Items]      
Beginning balance $ 4,153 $ 2,452 $ 2,858
Additions charged to expense 47,429 177  
Deductions   (207) (270)
Other 2,749 1,731 (136)
Ending balance $ 54,331 $ 4,153 $ 2,452