Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity (Details)

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Stockholders' Equity (Details) (USD $)
In Millions, except Share data
1 Months Ended 6 Months Ended 7 Months Ended 1 Months Ended
Oct. 11, 2011
Class A Common Stock [Member]
Aug. 31, 2011
Class A Common Stock [Member]
Oct. 11, 2011
Class A Common Stock [Member]
Apr. 30, 2011
Class A and Class B Common Stock [Member]
Stockholders' Equity (Textuals)        
Maximum authorized repurchase of common stock       $ 500.0
Common stock repurchased, shares 4,751,600 9,766,888 14,518,488  
Common stock repurchased $ 85.7 $ 187.5 $ 273.2  
Average cost of common stock repurchased $ 18.02 $ 19.20 $ 18.82