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Changes in Gross Unrecognized Tax Benefits (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Contingency [Line Items]      
Beginning balance $ 3,695 $ 3,513 $ 3,719
Additions based on tax positions related to the current year 801 547 871
Reductions due to lapse of applicable statute of limitations (271) (365) (1,077)
Ending balance $ 4,225 $ 3,695 $ 3,513