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Note 9 - Fair Value - Fair Values for Investments (Details) - USD ($) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
U.S. Treasury trading securities |  |  |  |  | $ 83,100,805
U.S. Treasury trading securities |  |  | 6,406 |  |
U.S. Treasury trading securities |  |  | (132,191) |  |
U.S. Treasury securities available for sale |  |  | 44,354 |  |
U.S. Treasury securities available for sale |  |  | 245,494 |  |
Realized gain (loss) |  |  | 50,760 |  |
Unrealized gain (loss) | 206,306 |  | 113,303 |  |
Reported Value Measurement [Member] |  |  |  |  |
U.S. Treasury trading securities |  |  |  |  |
U.S. Treasury securities available for sale | 150,346,440 |  | 150,346,440 |  |
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |  |
U.S. Treasury trading securities |  |  |  |  |
U.S. Treasury securities available for sale | $ 150,346,440 |  | $ 150,346,440 |  |

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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