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Fair Value Measurements, Fair Value Measurements Using Significant Unobservable Inputs (Level 3) (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2017
Jan. 31, 2017
Dec. 31, 2016
Fair Value Measurements [Abstract]      
Financial instruments     $ 592,211
Novartis [Member]      
Novartis Future Stock Purchase [Abstract]      
Additional amount of common stock required to be purchased in the future   $ 50,000  
Fair value of potential future premium $ 5,000    
1 Percent Convertible Senior Notes [Member]      
Fair Value Measurements [Abstract]      
Interest rate on convertible senior notes 1.00%   1.00%
Significant Other Observable Inputs (Level 2) [Member] | 1 Percent Convertible Senior Notes [Member]      
Fair Value Measurements [Abstract]      
Fair value of convertible notes $ 730,400    
Significant Unobservable Inputs (Level 3) [Member]      
Fair Value Measurements [Abstract]      
Financial instruments     $ 0
Potential Future Premium [Member]      
Unobservable Inputs (Level 3) [Roll Forward]      
Beginning balance of Level 3 asset 0    
Value of the potential premium we will receive from Novartis at inception of the SPA 5,035    
Recurring fair value adjustment (1,438)    
Ending balance of Level 3 asset $ 3,597