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Stockholders' Equity Share Repurchases (Details) - USD ($)
$ / shares in Units, $ in Thousands
2 Months Ended 12 Months Ended
Feb. 28, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Feb. 27, 2018
Dec. 12, 2017
Jul. 28, 2016
Jan. 31, 2016
Equity, Class of Treasury Stock [Line Items]                
Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities   799,680 18,904 18,904        
Treasury Stock, Value, Acquired, Cost Method     $ 75,569 $ 85        
2016 Share Repurchase Plan [Member]                
Equity, Class of Treasury Stock [Line Items]                
Treasury Stock, Shares, Acquired     1,100,000          
Treasury Stock, Value, Acquired, Cost Method     $ 75,600          
Treasury Stock Acquired, Average Cost Per Share     $ 65.74          
Stock Repurchase Program, Authorized Amount               $ 100,000
Stock Repurchase Program, Number of Shares Authorized to be Repurchased               5,000,000
Share Repurchase Plan [Member]                
Equity, Class of Treasury Stock [Line Items]                
Stock Repurchase Program, Authorized Amount           $ 250,000 $ 125,000  
Stock Repurchase Program, Number of Shares Authorized to be Repurchased           6,000,000 3,000,000  
Subsequent Event [Member] | Share Repurchase Plan [Member]                
Equity, Class of Treasury Stock [Line Items]                
Treasury Stock, Value, Acquired, Cost Method $ 125,000              
Stock Repurchase Program, Authorized Amount         $ 375,000      
Stock Repurchase Program, Number of Shares Authorized to be Repurchased         10,000,000