Net Income Per Share (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net income per share
Net income, as reported, for basic net income per share	$ 16.9	$ 17.8	$ 56.7	$ 59.3
Plus: interest expense on convertible debt, net of tax	1.1	1.1	3.2	3.2
Net income for diluted net income per share	$ 18.0	$ 18.9	$ 59.9	$ 62.5
Weighted average common shares outstanding	33.8	33.4	33.6	33.3
Assumed stock options exercised and awards vested, based on the treasury stock method	0.3	0.4	0.5	0.5
Assumed conversion of convertible debt, based on the if-converted method	2.9	2.9	2.9	2.9
Weighted average shares assuming dilution	37.0	36.7	37.0	36.7
Antidilutive options excluded from computation of diluted net income per share	1.7	1.7	1.3	1.1