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Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Amortized cost	$ 353,774	$ 402,517
Gross unrealized gains		50
Gross unrealized losses	(1,799)	(1,482)
Fair value	351,975	401,085
Marketable securities
Schedule of Available-for-sale Securities
Fair value	346,800	388,948
Money Market Funds
Schedule of Available-for-sale Securities
Amortized cost	5,175	12,137
Fair value	5,175	12,137
Corporate Securities
Schedule of Available-for-sale Securities
Amortized cost	123,860	182,394
Gross unrealized gains		6
Gross unrealized losses	(590)	(917)
Fair value	123,270	181,483
Government Securities
Schedule of Available-for-sale Securities
Amortized cost	224,739	207,986
Gross unrealized gains		44
Gross unrealized losses	(1,209)	(565)
Fair value	223,530	207,465
Cash and Cash Equivalents
Schedule of Available-for-sale Securities
Fair value	$ 5,175	$ 12,137

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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