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Debt (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Sep. 30, 2015
Convertible Notes      
Convertible senior notes noncurrent $ 643,434 $ 619,893 [1]  
Number of scheduled trading days preceding relevant maturity date at which Notes are convertible regardless of other circumstances 2 days    
Carrying Amount      
Convertible Notes      
Convertible senior notes, total $ 643,434 619,893  
Convertible senior notes noncurrent 643,434 619,893  
Fair Value      
Convertible Notes      
Convertible senior notes, total $ 1,436,545 1,623,545  
4.75% Convertible Senior Notes due 2015      
Convertible Notes      
Interest rate of debt (as a percent)     4.75%
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 $ 336,613 324,031  
0.375% Convertible Senior Notes due 2018 | Fair Value      
Convertible Notes      
Convertible senior notes, total $ 711,551 807,422  
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 $ 306,821 295,862  
1.25% Convertible Senior Notes due 2020 | Fair Value      
Convertible Notes      
Convertible senior notes, total $ 724,994 $ 816,123  
[1] The condensed consolidated balance sheet at December 31, 2015 has been derived from the audited financial statements at that date.