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Common Stock (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 0 Months Ended 3 Months Ended 12 Months Ended 0 Months Ended 1 Months Ended 3 Months Ended 12 Months Ended
Apr. 30, 2013
Jul. 31, 2012
Sep. 30, 2011
Dec. 31, 2013
May 16, 2012
Aug. 02, 2011
Apr. 30, 2013
Class A common stock
Jul. 31, 2012
Class A common stock
Oct. 02, 2011
Class A common stock
Dec. 31, 2013
Class A common stock
Dec. 31, 2012
Class A common stock
May 16, 2012
Class A common stock
Aug. 02, 2011
Class A common stock
Dec. 31, 2013
Class B common stock
Dec. 31, 2012
Class B common stock
Common Stock                              
Common Stock, votes per share                   1 1     10 10
Common Stock conversion ratio, at the option of the holder       1                      
Shares of Common Stock reserved for issuance under stock-based compensation plans                   3,719,322          
Shares of Common Stock reserved for conversion                           6,489,290  
Options to purchase shares of Class A common stock $ 90.0           $ 90.0                
Number of shares authorized to be repurchased         2,000,000 1,000,000           2,000,000 1,000,000    
Number of shares repurchased   2,000,000 1,000,000 454,000       2,000,000 1,000,000 454,000          
Cost of shares repurchased   $ 65.8 $ 27.2 $ 23.0       $ 65.8 $ 27.2 $ 23.0