Earnings Per Common Share (Earnings Per Share Computation) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Earnings Per Common Share [Abstract]
Net income	$ 138.7	$ 131.9
Less: Income allocated to participating securities	2.7	2.6
Net income available to common shareholders	$ 136.0	$ 129.3
Weighted average common shares outstanding	84.1	85.3
Dilutive common share equivalents from share-based compensation plans	1.0	1.7
Weighted average common and common equivalent shares outstanding, assuming dilution	85.1	87.0