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Note 6 - Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Available for sale securities	$ 489,303,666	$ 479,843,448
US Treasury Securities [Member]
Available for sale securities	9,771,000	9,452,000
US Government Agencies Debt Securities [Member]
Available for sale securities	113,483,000	126,433,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available for sale securities	85,985,000	81,128,000
US States and Political Subdivisions Debt Securities [Member]
Available for sale securities	205,758,000	195,302,000
Corporate Debt Securities [Member]
Available for sale securities	74,307,000	67,528,000
Fair Value, Recurring [Member]
Available for sale securities	489,304,000	479,843,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Available for sale securities	9,771,000	9,452,000
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available for sale securities	479,533,000	470,391,000
Fair Value, Recurring [Member] | US Treasury Securities [Member]
Available for sale securities	9,771,000	9,452,000
Fair Value, Recurring [Member] | US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available for sale securities	9,771,000	9,452,000
Fair Value, Recurring [Member] | US Government Agencies Debt Securities [Member]
Available for sale securities	113,483,000	126,433,000
Fair Value, Recurring [Member] | US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available for sale securities	113,483,000	126,433,000
Fair Value, Recurring [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available for sale securities	85,985,000	81,128,000
Fair Value, Recurring [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Fair Value, Inputs, Level 2 [Member]
Available for sale securities	85,985,000	81,128,000
Fair Value, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member]
Available for sale securities	205,758,000	195,302,000
Fair Value, Recurring [Member] | US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available for sale securities	205,758,000	195,302,000
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Available for sale securities	74,307,000	67,528,000
Fair Value, Recurring [Member] | Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available for sale securities	$ 74,307,000	$ 67,528,000

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 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: 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=121553693&loc=d3e26610-111562

Reference 4: 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=121645371&loc=d3e27161-111563

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