Cash, Cash Equivalents and Investments - Recurring basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	$ 76,960	$ 53,723
Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Total	33,350	29,775
Recurring [Member] \| Certificates of Deposit [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents, fair value disclosure	7,817	21,184
Recurring [Member] \| Corporate Bonds [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	25,533	8,432
Recurring [Member] \| Corporate Equity Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities		159
Recurring [Member] \| Quoted Prices in Active Markets of Identical Assets (Level 1) [Member]
Assets, Fair Value Disclosure [Abstract]
Total		159
Recurring [Member] \| Quoted Prices in Active Markets of Identical Assets (Level 1) [Member] \| Corporate Equity Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities		159
Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Assets, Fair Value Disclosure [Abstract]
Total	33,350	29,616
Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Certificates of Deposit [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents, fair value disclosure	7,817	21,184
Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities	$ 25,533	$ 8,432

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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

+ 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=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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