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Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Income Taxes [Line Items]            
Income tax expense $ 1,294 $ 2,170 $ 4,086 $ 6,302    
Effective income tax rate     29.00% 32.60%    
Income tax expense decreased due to decrease in federal tax rate due to Tax Cuts and Jobs Act     $ 2,200      
Settlement from Tax Audit [Member]            
Income Taxes [Line Items]            
Tax benefit recognized           $ 1,300
Anticipated [Member]            
Income Taxes [Line Items]            
Effective income tax rate         31.00%