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Fair Value Measurements - Assets (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Amount of transfers of financial assets between Level 1 and Level 2 measures | $ 0 |
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Short term investments | 28,400 | $ 31,100
Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Short term investments | 22,909 |
Recurring | Level 1 | Available-for-sale equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Short term investments | 22,909 |
Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Short term investments | 5,460 |
Recurring | Level 2 | Available-for-sale equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Short term investments | 5,450 |
Recurring | Level 2 | Mortgage-CMO debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis |  |
Short term investments | $ 10 |

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- Definition 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=75030427&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=82897076&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=82896770&loc=d3e26610-111562 + Details
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