Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity (Narratives) (Details)

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Stockholders' Equity (Narratives) (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Common Stock      
Shares repurchased value   $ 5,912 $ 47,655
Other Nonoperating Income (Expense)      
Share-based Compensation      
Share-based compensation expense   13,700 $ 7,800
Restricted Stock      
Share-based Compensation      
Unrecognized share-based compensation expense   $ 14,200  
Weighted average period for recognition of unvested shares   1 year 5 months 5 days  
Restricted Stock | Stock Option Plan      
Share-based Compensation      
Granted (USD per share)   $ 22.94 $ 17.68
Fair value of awards vesting during period   $ 9,700 $ 4,500
Restricted Stock | Maximum      
Share-based Compensation      
Vesting period   3 years  
Common Stock Class A      
Share-based Compensation      
Number of shares authorized   15,700,000.0  
Common Stock Class A | Stock Option Plan      
Share-based Compensation      
Number of shares available for grant   1,000,000  
Stock Buyback Program | Common Stock Class A      
Common Stock      
Shares repurchased 1,800,000 200,000 3,000,000
Shares repurchased value $ 27,100 $ 4,000 $ 46,500