Shareholders' Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended			3 Months Ended
	Apr. 27, 2011	Mar. 31, 2012	Apr. 13, 2012	Dec. 31, 2011	Mar. 31, 2011 Class A Common Stock [Member]	Mar. 07, 2012 Class A Common Stock [Member]	Mar. 31, 2012 Common Stock Equivalents 1.875% Notes [Member]	Mar. 31, 2012 Common Stock Equivalents 4.0% Notes [Member]
Class of Stock [Line Items]
Interest rate for senior notes							1.875%	4.00%
Stock repurchase authorized		$ 150.0
Terminated stock repurchase program	250.0				250.0
New stock repurchase authorized program	300.0	300.0
Common stock, share, issued		115,974,000	6,005,000	109,675,000		6,005,000
Common stock, par value		$ 0.01		$ 0.01
Per share price of common stock issued						$ 47.30
Proceeds from sale of stock, net of related fees						283.9
Pursuant to underwriting agreement, period in which the company cannot sell, pledge, or otherwise dispose of shares, in days		45
Stock repurchased and retired, shares					1,796,694
Stock repurchased and retired					$ 75.0