Summary of Significant Accounting Policies (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share ("EPS")
Increase in denominator representing dilutive effect of unvested restricted shares and stock options outstanding (in shares)	2,218,022		2,164,373	1,490,744
Earnings Per Share
Option to purchase shares of common stock excluded from the computation of diluted EPS (in shares)		1,500,115
Supplemental Cash Flow Information
Cash paid for interest			$ 2.6	$ 1.9
Cash paid for income taxes			19.2	22.5
Excess tax benefit received from the exercise of non-qualified stock options included in operating cash outflow			2.3	0.4
Excess tax benefit received from the exercise of non-qualified stock options included in financing cash inflows			$ 2.3	$ 0.4
Stock Options
Earnings Per Share
Option to purchase shares of common stock excluded from the computation of diluted EPS (in shares)	213,695	3,499,610	214,112	3,205,785
Exercise price of options to purchase common stock, low end of range (in dollars per share)	$ 77.72	$ 46.40	$ 73.32	$ 46.70
Exercise price of options to purchase common stock, high end of range (in dollars per share)	$ 94.34	$ 94.34	$ 94.34	$ 94.34