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Income Taxes - Reconciliation of Beginning and Ending Amount of Uncertain Tax Positions (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Balance at beginning of year $ 657 $ 259 $ 657
Additions based on tax positions related to the current year 11 454 70
Additions based on tax positions of prior years   4 13
Reductions for tax positions of prior years (50) (60) (443)
Settlements     (38)
Balance at end of year $ 618 $ 657 $ 259