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Fair Value Measurements (Fair Value of Marketable Securities) (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Fair Value Measurements |  |
Estimated fair value of marketable securities | $ 25,493 | $ 25,706
Fair value, inputs, Level 1 [Member] |  |
Fair Value Measurements |  |
Estimated fair value of marketable securities | 25,493 | 25,706
Fair value, inputs, Level 2 [Member] |  |
Fair Value Measurements |  |
Estimated fair value of marketable securities | 0 | 0
Fair value, inputs, Level 3 [Member] |  |
Fair Value Measurements |  |
Estimated fair value of marketable securities | $ 0 | $ 0

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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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