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, 2018
USD ($)
Debt Securities, Available-for-sale [Line Items]  
Fair Value $ 890,307
Gross Unrealized Losses, less than 12 months (8,330) [1]
Gross Unrealized Losses, 12 months or more 7,900
Corporate Debt Securities [Member]  
Debt Securities, Available-for-sale [Line Items]  
Fair Value 497,833
Gross Unrealized Losses, less than 12 months (4,702) [1]
U.S. Treasury securities  
Debt Securities, Available-for-sale [Line Items]  
Fair Value 211,588
Gross Unrealized Losses, less than 12 months (2,094) [1]
U.S. Government agency securities  
Debt Securities, Available-for-sale [Line Items]  
Fair Value 169,967
Gross Unrealized Losses, less than 12 months (1,445) [1]
Sovereign securities  
Debt Securities, Available-for-sale [Line Items]  
Fair Value 10,919
Gross Unrealized Losses, less than 12 months $ (89) [1]
[1] As of December 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 $7.9 million.