Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Effective tax rate		24.40%	36.50%	23.40%	35.20%
Effective statutory tax rate		21.00%		21.00%		35.00%
Decrease in excess tax benefits on restricted stock vestings				$ 1.1
Reduced to provisional amounts				$ 0.3
Provisional estimate of one-time deemed repatriation transition tax	$ 3.3
Deferred tax asset provisional write-down income tax expense benefit	$ 0.7