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Fair Value Measurements (Additional Disclosures Regarding Fair Value Measurements) (Details) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
Apr. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Senior Notes, Carrying Value $ 741,825,000 $ 2,127,474,000  
Level 2 | Fair Value      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Closing trading price per $100 as of the last day of trading for the year 100    
Convertible Senior Notes, Fair Value 1,648,567,000    
Convertible Senior Notes      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Convertible Senior Notes, Carrying Value 1,155,445,000 $ 1,386,324,000 $ 1,440,000,000
Senior Notes      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Senior Notes, Carrying Value 741,825,000    
Senior Notes | Level 2 | Fair Value      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Senior Notes, Fair Value $ 717,375,000