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Marketable Securities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Available-for-Sale Securities [Line Items]      
Amortized Cost $ 2,949,502 $ 2,434,145  
Unrealized Gain 1,049 819  
Unrealized Loss (4,793) (2,769)  
Debt securities due in one year or less 1,722,553 1,388,537  
Debt securities due after one year through three years 851,547 843,605  
Total debt securities $ 2,574,100 $ 2,232,142  
Realized investment gains (losses), description Realized gains and losses on sales of investments were not material in 2017, 2016 and 2015. Realized gains and losses on sales of investments were not material in 2017, 2016 and 2015. Realized gains and losses on sales of investments were not material in 2017, 2016 and 2015.
Cash Equivalents [Member]      
Schedule of Available-for-Sale Securities [Line Items]      
Amortized Cost $ 194,377 $ 124,793  
Unrealized Gain   1  
Unrealized Loss (1)    
Fair Value 194,376 124,794  
Investments [Member]      
Schedule of Available-for-Sale Securities [Line Items]      
Amortized Cost 2,755,125 2,309,352  
Unrealized Gain 1,049 818  
Unrealized Loss (4,792) (2,769)  
Fair Value 2,751,382 2,307,401  
Fair Value Measurements, Recurring      
Schedule of Available-for-Sale Securities [Line Items]      
Fair Value 2,945,758 2,432,195  
US Treasury Securities      
Schedule of Available-for-Sale Securities [Line Items]      
Amortized Cost 593,599 570,695  
Unrealized Gain 82 253  
Unrealized Loss (1,693) (635)  
US Treasury Securities | Fair Value Measurements, Recurring      
Schedule of Available-for-Sale Securities [Line Items]      
Fair Value 591,988 570,313  
Foreign Government Debt Securities      
Schedule of Available-for-Sale Securities [Line Items]      
Amortized Cost 6,982 17,999  
Unrealized Loss (30) (8)  
Foreign Government Debt Securities | Fair Value Measurements, Recurring      
Schedule of Available-for-Sale Securities [Line Items]      
Fair Value 6,952 17,991  
Corporate Debt Securities      
Schedule of Available-for-Sale Securities [Line Items]      
Amortized Cost 1,977,329 1,645,468  
Unrealized Gain 897 496  
Unrealized Loss (3,066) (2,126)  
Corporate Debt Securities | Fair Value Measurements, Recurring      
Schedule of Available-for-Sale Securities [Line Items]      
Fair Value 1,975,160 1,643,838  
Time Deposits      
Schedule of Available-for-Sale Securities [Line Items]      
Amortized Cost 371,515 199,906  
Unrealized Loss (4)    
Time Deposits | Fair Value Measurements, Recurring      
Schedule of Available-for-Sale Securities [Line Items]      
Fair Value 371,511 199,906  
Equity Securities      
Schedule of Available-for-Sale Securities [Line Items]      
Amortized Cost 77 77  
Unrealized Gain 70 70  
Equity Securities | Fair Value Measurements, Recurring      
Schedule of Available-for-Sale Securities [Line Items]      
Fair Value $ 147 $ 147