Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Basic net income per common share:
Net income	$ 43.6	$ 39.6
Weighted average common shares outstanding (in shares)	100.4	99.8
Basic net income per common share (in dollars per share)	$ 0.43	$ 0.40
Diluted net income per common share:
Net income	$ 43.6	$ 39.6
Weighted average common shares outstanding - Basic (in shares)	100.4	99.8
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.7	0.7
Stock options (in shares)	1.0	1.2
Weighted average common shares outstanding - Dilutive (in shares)	102.1	101.7
Diluted net income per common share (in dollars per share)	$ 0.43	$ 0.39
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)	0.8	0.5