Stockholders' Equity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended			36 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011
Summary of cash dividends paid
Total Cash Dividends Paid	$ 87.4	$ 79.7	$ 75.7	$ 242.8
Dividends per common share	$ 0.52	$ 0.50	$ 0.48