Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Amortized cost	$ 378,805	$ 402,517
Gross unrealized gains	9	50
Gross unrealized losses	(1,240)	(1,482)
Fair value	377,574	401,085
Marketable securities
Schedule of Available-for-sale Securities
Fair value	367,511	388,948
Money Market Funds
Schedule of Available-for-sale Securities
Amortized cost	10,063	12,137
Fair value	10,063	12,137
Corporate Securities
Schedule of Available-for-sale Securities
Amortized cost	154,634	182,394
Gross unrealized gains	2	6
Gross unrealized losses	(538)	(917)
Fair value	154,098	181,483
Government Securities
Schedule of Available-for-sale Securities
Amortized cost	214,108	207,986
Gross unrealized gains	7	44
Gross unrealized losses	(702)	(565)
Fair value	213,413	207,465
Cash and Cash Equivalents
Schedule of Available-for-sale Securities
Fair value	$ 10,063	$ 12,137

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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