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Fair Value of Financial Instruments - Carrying Value (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Other non-current assets:    
Carrying value of the convertible notes $ 329,462  
Total Fair Value | Recurring    
Assets:    
Cash and cash equivalents 192,248 $ 100,304
Marketable securities    
Corporate debt securities 163,770 39,736
Long term marketable securities:    
Corporate debt securities 415,650 132,477
Government debt securities 3,148 1,161
Other non-current assets:    
Marketable securities - restricted (SERP) 326 335
Total assets at fair value 775,142 274,013
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring    
Assets:    
Cash and cash equivalents 192,248 100,304
Marketable securities    
Corporate debt securities 245 2,118
Long term marketable securities:    
Corporate debt securities 445 448
Other non-current assets:    
Marketable securities - restricted (SERP) 1  
Total assets at fair value 192,939 102,870
Significant Other Observable Inputs (Level 2) | Recurring    
Marketable securities    
Corporate debt securities 163,525 37,618
Long term marketable securities:    
Corporate debt securities 415,205 132,029
Government debt securities 3,148 1,161
Other non-current assets:    
Marketable securities - restricted (SERP) 325 335
Total assets at fair value 582,203 $ 171,143
0.625% Convertible Senior Notes due 2023    
Other non-current assets:    
Amount issued 402,500  
0.625% Convertible Senior Notes due 2023 | Significant Other Observable Inputs (Level 2)    
Other non-current assets:    
Carrying value of the convertible notes 329,500  
Amount issued 402,500  
Estimated fair value $ 375,800