Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Basic net income per common share:
Net income	$ 68.1	$ 50.1
Weighted average common shares outstanding (in shares)	96.2	98.5
Basic net income per common share ( in dollars per share)	$ 0.71	$ 0.51
Diluted net income per common share:
Net income	$ 68.1	$ 50.1
Weighted average common shares outstanding (in shares)	96.2	98.5
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.6	0.7
Stock options (in shares)	1.1	1.4
Weighted average common shares outstanding - Dilutive (in shares)	97.9	100.6
Diluted net income per common share (in dollars per share)	$ 0.70	$ 0.50