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Stockholders' Equity (Narrative) (Details)
 1 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Feb. 29, 2016
USD ($)
Jul. 20, 2012
shares
Aug. 31, 2011
shares
Class of Stock [Line Items]              
Shares repurchased in association with tax withholdings on the vesting of RSUs (in shares) | shares   32,457 94,826 77,649      
Net premiums paid $ (33,400,000)            
Class A common stock              
Class of Stock [Line Items]              
Conversion of common stock (in shares) | shares   1          
Common stock number of votes   1          
Number of shares authorized to be repurchased (in shares) | shares           1,000,000 2,000,000
Amount authorized to be repurchased         $ 250,000,000    
Class A common stock | Restricted Stock Units (RSUs)              
Class of Stock [Line Items]              
Treasury stock acquired, average cost (in dollars per share) | $ / shares   $ 99.40          
Adjustments related to tax withholding for share-based compensation   $ 3,200,000          
Class A common stock | The 2016 Stock Repurchase Authorization              
Class of Stock [Line Items]              
Remaining authorized repurchase amount $ 162,600,000 $ 162,600,000          
Class B common stock              
Class of Stock [Line Items]              
Percentage of Class B common to total common stock subject to automatic conversion   1.00%          
Common stock number of votes   10