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Stockholders' Equity (Schedule of Stock Repurchased and Retired) (Details) - Class A common stock - $ / shares
9 Months Ended 32 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Equity, Class of Treasury Stock [Line Items]    
Shares repurchased pursuant to repurchase authorizations (in shares) 1,199,257 2,241,982
Average purchase price (in dollars per share) $ 93.11 $ 88.81