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Fair Value Measurements (Additional Information Regarding Fair Value Measurements) (Details) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Nov. 15, 2017
Dec. 31, 2014
Level 2 [Member] | Fair Value [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Convertible Senior Notes, Fair Value $ 2,059,942,000      
Senior Notes Due 2019 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Debt instrument, face amount 1,000      
Convertible debt 1,406,157,000 $ 1,386,324,000   $ 1,440,000,000
Senior Notes Due 2019 [Member] | Level 2 [Member] | Fair Value [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Closing trading price per $100 as of the last day of trading for the quarter 100      
Unsecured Debt [Member] | 2027 Notes [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Debt instrument, face amount     $ 750,000,000  
Senior Notes [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Long-term Debt, Carrying Value 741,277,000      
Senior Notes [Member] | Level 2 [Member] | Fair Value [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Long-term Debt, Fair Value $ 727,778,000