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Stockholders' Equity (Schedule of Stock Repurchased and Retired) (Details) - Class A common stock - $ / shares
3 Months Ended 26 Months Ended
Mar. 31, 2018
Mar. 31, 2018
Equity, Class of Treasury Stock [Line Items]    
Shares repurchased pursuant to repurchase authorizations (in shares) 50,000 1,092,725
Average purchase price (in dollars per share) $ 99.02 $ 84.55