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Cash and Cash Equivalents and Investments (Details 1) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents
Fair Value
Amortized Cost	$ 20,421	$ 61,661
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	20,421	61,661
U.S. Treasury Securities
Fair Value
Amortized Cost	214,302	222,316
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(317)	(168)
Estimated Fair Value	213,985	222,148
Corporate Debt Securities
Fair Value
Amortized Cost	27,943	27,033
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(77)	(54)
Estimated Fair Value	27,866	26,979
Total Short-term Investments
Fair Value
Amortized Cost	242,245	249,349
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(394)	(222)
Estimated Fair Value	241,851	249,127
Total Cash and Cash Equivalents and Investments
Fair Value
Amortized Cost	262,666	311,010
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(394)	(222)
Estimated Fair Value	$ 262,272	$ 310,788

X

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ References

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=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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