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Income Taxes - Summary of Deferred Tax Asset Valuation Allowance (Detail) - Valuation Allowance of Deferred Tax Assets - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Valuation And Qualifying Accounts Disclosure [Line Items]      
Beginning balance $ 17,807 $ 21,052 $ 21,232
Additions charged to cost and expenses 324 1,862  
Deductions (6,556) (5,107) (180)
Ending balance $ 11,575 $ 17,807 $ 21,052