Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator
Net income	$ 60.4	$ 47.6
Denominator
Basic weighted-average common shares outstanding	91.2	91.0
Dilutive shares	0.8	0.2
Weighted-average common and common equivalent shares outstanding - diluted	92.0	91.2
Per common share income
Net income, per basic share	$ 0.66	$ 0.52
Net income, per diluted share	$ 0.66	$ 0.52
Antidilutive stock options and stock awards	0.0	0.8