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Marketable Securities - Unrealized losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair value
Fair value, less than 12 months	$ 207,603	$ 99,788
Fair value, 12 months or greater	139,197	238,684
Unrealized losses
Unrealized losses, Less than 12 months	(598)	(149)
Unrealized losses, 12 months or greater	(1,201)	(1,333)
Corporate Securities
Fair value
Fair value, less than 12 months	79,290	82,215
Fair value, 12 months or greater	43,980	88,990
Unrealized losses
Unrealized losses, Less than 12 months	(137)	(133)
Unrealized losses, 12 months or greater	(453)	(784)
Government Securities
Fair value
Fair value, less than 12 months	128,313	17,573
Fair value, 12 months or greater	95,217	149,694
Unrealized losses
Unrealized losses, Less than 12 months	(461)	(16)
Unrealized losses, 12 months or greater	$ (748)	$ (549)

X

- Definition

Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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- Definition

Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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