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Stockholders' Equity (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2018
Dec. 31, 2015
Jan. 21, 2015
Class of Stock [Line Items]                
Stock Repurchased and Retired During Period, Value   $ (40,000)   $ (40,000)   $ (37,456) $ (80,000)  
Number of shares authorized to be repurchased under the plan               20,000,000.0
UpfrontPaymentUnderAcceleratedStockRepurchaseProgram   $ 50,000   $ 50,000     $ 100,000  
Stock Repurchased and Retired During Period, Shares (700,000) (3,100,000) (830,000) (3,187,000) (736,000)   (7,812,000)  
UnsettledForwardContractIndexedtoIssuersStockClassifiedwithinStock   $ 10,000   $ 10,000     $ 20,000  
Remaining shares authorized to be repurchased           3,600,000