Income Tax Provision - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Effective income tax rates	(0.80%)		12.40%	(0.10%)	12.50%
Excess tax benefit	$ 9.3		$ 9.0	$ 30.0	$ 27.6
Effective income tax rate excluding excess tax benefit	23.30%		32.80%	25.80%	35.90%
Change in tax rate, income tax benefit		$ 26.0
Measurement period adjustment, income tax expense				$ 27.7
Income tax payments				$ 6.8	$ 29.0