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Income Taxes - Summary of Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]        
Balance at beginning of period   $ 874 $ 777 $ 124
Additions based on tax positions related to the current year     162 118
Additions (reductions) based on tax positions related to prior years       537
Additions (reductions) based on tax positions related to prior years $ (100) (78)    
Reductions due to statute of limitations expiring $ (300) (301) (173) (2)
Revaluation due to Tax Reform     108  
Balance at end of period   $ 495 $ 874 $ 777