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Stockholders' Equity (Narrative) (Details) - Class A common stock - USD ($)
6 Months Ended 17 Months Ended
Jun. 30, 2017
Jun. 30, 2017
Feb. 29, 2016
Equity, Class of Treasury Stock [Line Items]      
Authorized repurchase amount     $ 250,000,000
Restricted stock units (RSUs)      
Equity, Class of Treasury Stock [Line Items]      
Repurchase related to stock award tax withholdings (in shares) 32,143    
Average purchase price per share (in dollars per share) $ 99.32    
Repurchase related to stock award tax withholdings $ 3,200,000    
2016 Share Repurchase Authorization      
Equity, Class of Treasury Stock [Line Items]      
Remaining authorized repurchase amount $ 171,400,000 $ 171,400,000  
Average purchase price per share (in dollars per share) $ 87.94 $ 81.85