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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

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