Income Taxes - Schedule of Provision for Income Taxes and Effective Income Tax Rates (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Earnings before income taxes	$ 67.7	$ 7.4	$ 106.1	$ 55.4
Income tax expense	$ 19.7	$ 2.1	$ 31.5	$ 22.0
Effective income tax rate	29.10%	28.40%	29.70%	39.70%
Earnings before income taxes, $ Change	$ 60.3		$ 50.7
Income tax expense, $ Change	$ 17.6		$ 9.5
Earnings before income taxes, % Change	814.90%		91.50%
Income tax expense, % Change	838.10%		43.20%