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Income Taxes - Additional Information (Detail) - USD ($)
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]          
Effective income tax rate 27.50% 31.60% 27.40% 29.50%  
Reduction in corporate tax rate     21.00%   35.00%
Adjustment to income tax expense     $ 2,300,000    
Minimum [Member]          
Income Taxes [Line Items]          
Reasonably possible decrease in unrecognized tax benefits $ 11,000,000   11,000,000    
Maximum [Member]          
Income Taxes [Line Items]          
Reasonably possible decrease in unrecognized tax benefits $ 14,000,000   $ 14,000,000