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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, 2017
Income Taxes [Line Items]          
Provision for (benefit from) income taxes $ 1,953 $ 918 $ 1,708 $ 890  
Effective income tax rate 11.10% 5.50% 4.60% 1.70%  
U.S. statutory tax rate     21.00%   35.00%