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Investments and Fair Value Measurements- Recurring basis (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Assets, Fair Value Disclosure [Abstract] |  |
Investments, amortized cost | $ 27,706 | $ 32,608
Recurring [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Total | 27,706 | 33,350
Recurring [Member] | Certificates of Deposit [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Cash and cash equivalents, fair value disclosure | 7,299 | 7,817
Recurring [Member] | Corporate Bonds [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Investments, amortized cost | 20,407 | 25,533
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Total | 27,706 | 33,350
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 | 7,299 | 7,817
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Investments, amortized cost | $ 20,407 | $ 25,533

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References 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 + Details
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- Definition Amount of debt 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 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -Subparagraph (b)-(g) -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803 + Details
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- Definition 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 + Details
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