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Stockholders' Equity - Treasury Stock (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 11, 2017
Treasury Stock        
Authorized amount of common stock repurchase (in dollars)       $ 100,000
Purchase of common stock $ 3,018 $ 13,035 $ 9,222  
Accrued expenses and other current liabilities        
Treasury Stock        
Authorized amount of common stock repurchase (in dollars) $ 100