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Long-Term Obligations and Commitments, Convertible Notes (Details)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Dec. 31, 2016
USD ($)
Nov. 30, 2014
USD ($)
Dec. 31, 2018
USD ($)
shares
$ / shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Mar. 31, 2017
Convertible Notes [Abstract]            
Loss on early retirement of debt     $ 0 $ 0 $ (3,983)  
Amortization of debt discount and debt issuance costs     35,200 32,500 $ 25,100  
Long-term debt     $ 568,215 $ 533,111    
1 Percent Convertible Senior Notes [Member]            
Convertible Notes [Abstract]            
Maturity date     Nov. 30, 2021      
Interest rate 1.00%   1.00% 1.00% 1.00% 1.00%
Conversion price per share (in dollars per share) | $ / shares     $ 66.81      
Total shares of common stock subject to conversion (in shares) | shares     10,300,000      
Percentage of principal amount used a purchase price upon occurrence of fundamental change     100.00%      
Principal amount of convertible notes outstanding     $ 685,450 $ 685,450    
Unamortized portion of debt discount     110,817 144,112    
Long-term debt     568,215 533,111    
Carrying value of equity component     219,011 219,011    
1 Percent Convertible Senior Notes [Member] | Level 2 [Member]            
Convertible Notes [Abstract]            
Fair value of outstanding notes     $ 724,966 $ 727,420    
1 Percent Convertible Senior Notes Issued in November 2014 [Member]            
Convertible Notes [Abstract]            
Face amount of offering   $ 500,000        
Proceeds from issuance of 1 percent convertible senior notes, net of issuance costs   487,000        
Issue date     Nov. 30, 2014      
Effective interest rate     7.80%      
Amortization period of debt discount     7 years      
1 Percent Convertible Senior Notes Issued in November 2014 [Member] | Borrowing Rate [Member]            
Convertible Notes [Abstract]            
Debt measurement input rate     0.074      
1 Percent Convertible Senior Notes Issued in December 2016 [Member]            
Convertible Notes [Abstract]            
Face amount of offering $ 185,500       $ 185,500  
Issue date     Dec. 31, 2016      
Effective interest rate     7.20%      
Amortization period of debt discount     5 years      
1 Percent Convertible Senior Notes Issued in December 2016 [Member] | Borrowing Rate [Member]            
Convertible Notes [Abstract]            
Debt measurement input rate     0.068      
2 3/4 Percent Convertible Senior Notes [Member]            
Convertible Notes [Abstract]            
Principal amount repurchased 61,100 140,000     $ 61,100  
Repurchase of convertible senior notes   441,900        
Loss on early retirement of debt $ 3,983          
Interest rate 2.75%   2.75% 2.75% 2.75% 2.75%
Principal amount of convertible notes outstanding   $ 61,200