Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income									$ 209.4	$ 187.9	$ 164.3
Basic net income per common share:
Weighted average common shares outstanding (in shares)									96.8	100.0	100.2
Basic net income per common share ( in dollars per share)	$ 0.55	$ 0.58	$ 0.52	$ 0.51	$ 0.47	$ 0.49	$ 0.49	$ 0.43	$ 2.16	$ 1.88	$ 1.64
Diluted net income per common share:
Weighted average common shares outstanding (in shares)									96.8	100.0	100.2
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)									0.7	0.7	0.8
Stock options (in shares)									1.2	1.4	1.0
Weighted average common shares outstanding - Dilutive (in shares)									98.7	102.1	102.0
Diluted net income per common share (in dollars per share)	$ 0.54	$ 0.57	$ 0.51	$ 0.50	$ 0.46	$ 0.48	$ 0.48	$ 0.43	$ 2.12	$ 1.84	$ 1.61
Anti-dilutive shares outstanding, excluded from computation (in shares)									0.2		0.5