Stock-Based Compensation Plans (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Common stock authorized for issuance under the Incentive Plan, shares	10,000,000
Period of expiration of stock options granted to employees and non-employee directors, in years, maximum	10
Period of expiration of stock options granted to employees, in years	7	7	7
Period of expiration of stock options granted to non-employee directors, in years	5	5	5
Total fair value of restricted share awards vested during the period	$ 4.2	$ 1.9	$ 2.8
Income tax benefits realized	5.1	6.3	1.0
Stock Option Awards [Member]
Weighted-average estimated grant-date fair values of employee and director stock options granted (per share)	$ 28.10	$ 16.64	$ 7.28
Total pre-tax intrinsic value of stock options exercised	12.2	16.6	3.8
Pre-tax unrecognized compensation expense, net of estimated forfeitures	3.7
Weighted-average period of pre-tax unrecognized compensation expense for stock options to be recognized as expense, years	1.8
Restricted Share Awards [Member]
Pre-tax unrecognized compensation expense, net of estimated forfeitures	$ 3.5
Weighted-average period of pre-tax unrecognized compensation expense for stock options to be recognized as expense, years	2.0
Period restricted share awards generally cliff vest after date of grant, years	3