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Debt (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Dec. 31, 2015
Convertible Notes    
Convertible senior notes noncurrent $ 627,642 $ 619,893 [1]
Carrying Amount    
Convertible Notes    
Convertible senior notes, total 627,642 619,893
Convertible senior notes noncurrent 627,642 619,893
Fair Value    
Convertible Notes    
Convertible senior notes, total $ 1,163,349 1,623,545
0.375% Convertible Senior Notes due 2018 and 1.25% Convertible Senior Notes due 2020    
Convertible Notes    
Number of days within 30 consecutive trading days in which the price of the entity's common stock must exceed the conversion price for the notes to be converted 20 days  
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible 30 days  
Percentage of the closing sales price of common stock that the conversion price must exceed in order for the notes to be convertible (as a percent) 130.00%  
Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period 5 days  
Number of consecutive trading days before five consecutive business days during the note measurement period 5 days  
Conversion ratio, principal amount of note 1,000  
Percentage of the trading price to the product of the last reported sale price of the common stock and the conversion rate, maximum (as a percent) 98.00%  
0.375% Convertible Senior Notes due 2018    
Convertible Notes    
Interest rate of debt (as a percent) 0.375%  
0.375% Convertible Senior Notes due 2018 | Carrying Amount    
Convertible Notes    
Convertible senior notes, total $ 328,173 324,031
0.375% Convertible Senior Notes due 2018 | Fair Value    
Convertible Notes    
Convertible senior notes, total $ 574,922 807,422
1.25% Convertible Senior Notes due 2020    
Convertible Notes    
Interest rate of debt (as a percent) 1.25%  
1.25% Convertible Senior Notes due 2020 | Carrying Amount    
Convertible Notes    
Convertible senior notes, total $ 299,469 295,862
1.25% Convertible Senior Notes due 2020 | Fair Value    
Convertible Notes    
Convertible senior notes, total $ 588,427 $ 816,123
[1] The condensed consolidated balance sheet at December 31, 2015 has been derived from the audited financial statements at that date.