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Income Taxes - Gross Unrecognized Tax Benefit (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]    
Unrecognized tax benefit beginning balance $ 4,097 $ 3,586
Increase as a result of tax positions taken during the period 4,353 2,354
Decrease as a result of tax positions taken during the period (2,311) (1,356)
Decrease as a result of expiring statutes (1,689) (487)
Unrecognized tax benefit ending balance $ 4,450 $ 4,097