Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Initial analysis of tax law changes recorded net tax expense				$ 4.3
Provisional expense for Transition Tax				55.0
Revaluation of deferred tax assets and liabilities				50.7
Provisional benefit to net deferred taxes		$ 3.2				$ 24.6
Provisional transition tax on unrepatriated foreign earnings	$ 3.3			$ 13.4		(51.8)
Effective income tax rate (as a percent)	16.20%		15.70%	17.20%	23.40%
Reduction in liability for certain tax provisions	$ 2.4
Favorable tax impact	1.1
Liability for uncertain tax positions	4.5			$ 4.5		6.9
Unrecognized tax benefits if recognized that would affect effective tax rate	3.3			3.3		4.4
Reduction in accrued interest expense	0.3
Accrued interest related to uncertain tax positions	0.6			0.6		0.7
Accrued penalties related to uncertain tax positions	0.1			0.1		$ 0.1
Unrecognized tax benefits subject to change within the next 12 months	$ 4.5			$ 4.5