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
Dec. 31, 2017
USD ($)
Investments in an Unrealized Loss Position  
Fair Value $ 1,542,909
Gross Unrealized Losses, less than 12 months (9,866) [1]
Gross Unrealized Losses, 12 months or more 5,300
Corporate debt securities  
Investments in an Unrealized Loss Position  
Fair Value 792,436
Gross Unrealized Losses, less than 12 months (4,245) [1]
U.S. Treasury securities  
Investments in an Unrealized Loss Position  
Fair Value 404,982
Gross Unrealized Losses, less than 12 months (2,941) [1]
U.S. Government agency securities  
Investments in an Unrealized Loss Position  
Fair Value 334,064
Gross Unrealized Losses, less than 12 months (2,611) [1]
Sovereign securities  
Investments in an Unrealized Loss Position  
Fair Value 11,427
Gross Unrealized Losses, less than 12 months $ (69) [1]
[1] As of December 31, 2017, the amount of total gross unrealized losses related to investments that had been in a continuous loss position for 12 months or more was $5.3 million.