Summary of Significant Accounting Policies (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share ("EPS")
Increase in denominator representing dilutive effect of unvested restricted shares and stock options outstanding (in shares)	1,445,369	1,461,566	1,428,765
Supplemental Cash Flow Information
Cash paid for interest	$ 4.6	$ 3.8	$ 0.6
Cash paid for income taxes	29.8	58.2	33.6
Excess tax benefit received from the exercise of non-qualified stock options included in operating cash outflow	1.1	0.9	1.6
Excess tax benefit received from the exercise of non-qualified stock options included in financing cash inflows	$ 1.1	$ 0.9	$ 1.6
Stock Options
Earnings Per Share
Option to purchase shares of common stock excluded from the computation of diluted EPS (in shares)	2,337,264	2,335,194	1,639,806
Exercise price of options to purchase common stock, low end of range (in dollars per share)	$ 49.20	$ 54.15	$ 51.93
Exercise price of options to purchase common stock, high end of range (in dollars per share)	$ 94.34	$ 94.34	$ 94.34