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Stockholders' Equity (Narrative) (Details) - USD ($)
9 Months Ended
Sep. 30, 2018
Feb. 29, 2016
Equity, Class of Treasury Stock [Line Items]    
Repurchase related to stock award tax withholdings (in shares) 29,875  
Class A common stock    
Equity, Class of Treasury Stock [Line Items]    
Authorized repurchase amount   $ 250,000,000
Class A common stock | Restricted stock units (RSUs)    
Equity, Class of Treasury Stock [Line Items]    
Average purchase price per share (in dollars per share) $ 112.19  
Repurchase related to stock award tax withholdings $ 3,400,000  
2016 Share Repurchase Authorization | Class A common stock    
Equity, Class of Treasury Stock [Line Items]    
Remaining authorized repurchase amount $ 50,900,000