Quarterly report pursuant to Section 13 or 15(d)

Marketable Securities - Continuous Unrealized Loss Position (Details)

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Marketable Securities - Continuous Unrealized Loss Position (Details)
$ in Thousands
Mar. 31, 2018
USD ($)
Investments in an Unrealized Loss Position  
Fair Value $ 1,499,818
Gross Unrealized Losses, less than 12 months (15,514) [1]
Gross Unrealized Losses, 12 months or more 6,300
Corporate debt securities  
Investments in an Unrealized Loss Position  
Fair Value 773,988
Gross Unrealized Losses, less than 12 months (8,407) [1]
U.S. Treasury securities  
Investments in an Unrealized Loss Position  
Fair Value 400,675
Gross Unrealized Losses, less than 12 months (3,866) [1]
U.S. Government agency securities  
Investments in an Unrealized Loss Position  
Fair Value 307,473
Gross Unrealized Losses, less than 12 months (3,100) [1]
Sovereign securities  
Investments in an Unrealized Loss Position  
Fair Value 17,682
Gross Unrealized Losses, less than 12 months $ (141) [1]
[1] As of March 31, 2018, the amount of total gross unrealized losses related to investments that had been in a continuous loss position for 12 months or more was $6.3 million.