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Stockholders' Equity (Schedule of Stock Repurchased and Retired) (Details) - Class A common stock - 2016 Share Repurchase Authorization - $ / shares
9 Months Ended 20 Months Ended
Sep. 30, 2017
Sep. 30, 2017
Equity, Class of Treasury Stock [Line Items]    
Shares repurchased pursuant to repurchase authorizations (in shares) 310,000 1,023,725
Average purchase price (in dollars per share) $ 91.33 $ 83.25