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Stockholders' Equity and Compensation Arrangements Repurchase Program (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
shares
Sep. 30, 2017
USD ($)
shares
Common Class A [Member]    
Employee Stock Purchase Plan [Line Items]    
Stock Repurchase Program, Authorized Amount | $ $ 700.0 $ 700.0
Stock Repurchased During Period, Shares | shares 2,384,200 16,906,334
Stock Repurchased During Period, Value | $ $ 9.2 $ 60.0
Common Stock [Member]    
Employee Stock Purchase Plan [Line Items]    
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased | shares 135,200,000 135,200,000