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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
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 Tax Disclosure [Abstract]          
Effective tax rate 37.00% 29.00% 47.00% 26.00%  
Federal statutory rate     21.00%   35.00%
Increase (decrease) in income tax due to change in fair value of contingent consideration $ 1.2 $ (1.0)   $ (3.0)