Net Income Per Share (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income, as reported, for basic net income per share	$ 127.1	$ 112.3	$ 80.7
Plus: interest expense on convertible debt, net of tax	0	0	2.0
Net income for diluted net income per share	127.1	112.3	82.7
Weighted average common shares outstanding (in shares)	70.9	69.6	68.1
Assumed stock options exercised and awards vested, based on the treasury stock method (in shares)	1.9	1.7	1.1
Assumed conversion of convertible debt, based on the if-converted method (in shares)	0	0.1	2.6
Weighted average shares assuming dilution (in shares)	72.8	71.4	71.8
Antidilutive options excluded from computation of diluted net income per share (in shares)	0.5		1.0
Stock Repurchase Program, Authorized Amount	$ 100.0