Income Tax Provision - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Effective income tax rates	12.10%	2.70%	12.30%	16.50%
Federal income tax rate	35.00%	35.00%	35.00%	35.00%
Exess tax benefit	$ 9.0	$ 13.3	$ 27.6	$ 20.8
Effective income tax rate excluding excess tax benefit	32.80%	39.30%	36.00%	38.50%
Income tax payments			$ 29.0	$ 25.0