Shareholders' Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	1 Months Ended	12 Months Ended					0 Months Ended	12 Months Ended
	May 20, 2010	Apr. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Apr. 27, 2011	Apr. 20, 2009	Apr. 27, 2011 Class A Common Stock [Member]	Dec. 31, 2011 Class A Common Stock [Member]	Dec. 31, 2009 Class A Common Stock [Member]	Nov. 03, 2009 Class A Common Stock [Member]	Dec. 31, 2007 Class A Common Stock [Member]	Dec. 31, 2011 Common Stock Equivalent 1.875% Notes [Member]	Dec. 31, 2011 Common Stock Equivalent 4.0% Notes [Member]
Class of Stock [Line Items]
Interest rates for senior notes													1.875%	4.00%
Stock repurchase authorized			$ 150.0								$ 250.0
Common stock shares repurchased and retired		2,000,000	5,917,940	3,174,385	52,000
Common stock repurchased and retired values		50.0	225.1	107.4	1.7
Closing stock price per share							$ 24.80
Debt instrument interest rate							4.00%
Terminated stock repurchase program						250.0
Stock available for termination						65.9
New stock repurchase authorized program			$ 300.0					$ 300.0
Shares registered for Performance and Equity Incentive Plan	15,000,000
Shares, issued												4,000,000
Shares, Outstanding										900,000
Shares reclassified as authorized and unissued									1,700,000