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Note 7 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($) | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value |  | $ 556,415,000
Temporarily impaired securities, less than 12 months, unrealized losses |  | 3,343,000
Temporarily impaired securities, 12 months or longer, fair value |  | 624,880,000
Temporarily impaired securities, 12 months or longer, unrealized losses |  | 13,341,000
Temporarily impaired securities, total fair value |  | 1,181,295,000
Temporarily impaired securities, total unrealized losses | $ 38,900,000 | 16,684,000
US Treasury Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 199,374,000 | 199,823,000
Temporarily impaired securities, less than 12 months, unrealized losses | 114,000 | 62,000
Temporarily impaired securities, 12 months or longer, fair value |  | 49,697,000
Temporarily impaired securities, 12 months or longer, unrealized losses |  | 295,000
Temporarily impaired securities, total fair value | 199,374,000 | 249,520,000
Temporarily impaired securities, total unrealized losses | 114,000 | 357,000
US Government Corporations and Agencies Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 5,208,000 | 5,711,000
Temporarily impaired securities, less than 12 months, unrealized losses | 160,000 | 70,000
Temporarily impaired securities, 12 months or longer, fair value | 1,304,000 |
Temporarily impaired securities, 12 months or longer, unrealized losses | 50,000 |
Temporarily impaired securities, total fair value | 6,512,000 | 5,711,000
Temporarily impaired securities, total unrealized losses | 210,000 | 70,000
US Government-sponsored Enterprises Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value |  |
Temporarily impaired securities, less than 12 months, unrealized losses |  |
Temporarily impaired securities, 12 months or longer, fair value | 383,447,000 | 390,336,000
Temporarily impaired securities, 12 months or longer, unrealized losses | 16,553,000 | 9,664,000
Temporarily impaired securities, total fair value | 383,447,000 | 390,336,000
Temporarily impaired securities, total unrealized losses | 16,553,000 | 9,664,000
Collateralized Mortgage Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 247,059,000 | 342,436,000
Temporarily impaired securities, less than 12 months, unrealized losses | 6,088,000 | 3,147,000
Temporarily impaired securities, 12 months or longer, fair value | 372,218,000 | 178,617,000
Temporarily impaired securities, 12 months or longer, unrealized losses | 15,935,000 | 3,112,000
Temporarily impaired securities, total fair value | 619,277,000 | 521,053,000
Temporarily impaired securities, total unrealized losses | 22,023,000 | 6,259,000
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value |  | 1,914,000
Temporarily impaired securities, less than 12 months, unrealized losses |  | 30,000
Temporarily impaired securities, 12 months or longer, fair value |  |
Temporarily impaired securities, 12 months or longer, unrealized losses |  |
Temporarily impaired securities, total fair value |  | 1,914,000
Temporarily impaired securities, total unrealized losses |  | 30,000
Collateralized Mortgage Obligations [Member] |  |
Temporarily impaired securities, less than 12 months, fair value |  | 1,516,000
Temporarily impaired securities, less than 12 months, unrealized losses |  | 17,000
Temporarily impaired securities, 12 months or longer, fair value | 1,086,000 |
Temporarily impaired securities, 12 months or longer, unrealized losses | 33,000 |
Temporarily impaired securities, total fair value | 1,086,000 | 1,516,000
Temporarily impaired securities, total unrealized losses | 33,000 | 17,000
Total Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 451,641,000 |
Temporarily impaired securities, less than 12 months, unrealized losses | 6,362,000 |
Temporarily impaired securities, 12 months or longer, fair value | 758,055,000 |
Temporarily impaired securities, 12 months or longer, unrealized losses | 32,571,000 |
Temporarily impaired securities, total fair value | 1,209,696,000 |
Temporarily impaired securities, total unrealized losses | 38,933,000 |
Corporate Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value |  | 5,015,000
Temporarily impaired securities, less than 12 months, unrealized losses |  | 17,000
Temporarily impaired securities, 12 months or longer, fair value |  |
Temporarily impaired securities, 12 months or longer, unrealized losses |  |
Temporarily impaired securities, total fair value |  | 5,015,000
Temporarily impaired securities, total unrealized losses |  | 17,000
Preferred Stock of Government Sponsored Entities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 6,397,000 |
Temporarily impaired securities, less than 12 months, unrealized losses | 3,706,000 |
Temporarily impaired securities, 12 months or longer, fair value |  |
Temporarily impaired securities, 12 months or longer, unrealized losses |  |
Temporarily impaired securities, total fair value | 6,397,000 |
Temporarily impaired securities, total unrealized losses | 3,706,000 |
Mutual Funds [Member] |  |
Temporarily impaired securities, less than 12 months, fair value |  |
Temporarily impaired securities, less than 12 months, unrealized losses |  |
Temporarily impaired securities, 12 months or longer, fair value | 6,034,000 | 6,230,000
Temporarily impaired securities, 12 months or longer, unrealized losses | 195,000 | 270,000
Temporarily impaired securities, total fair value | 6,034,000 | 6,230,000
Temporarily impaired securities, total unrealized losses | 195,000 | $ 270,000
Other Equity Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 1,008,000 |
Temporarily impaired securities, less than 12 months, unrealized losses | 679,000 |
Temporarily impaired securities, 12 months or longer, fair value |  |
Temporarily impaired securities, 12 months or longer, unrealized losses |  |
Temporarily impaired securities, total fair value | 1,008,000 |
Temporarily impaired securities, total unrealized losses | 679,000 |
Total Equity Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 7,405,000 |
Temporarily impaired securities, less than 12 months, unrealized losses | 4,385,000 |
Temporarily impaired securities, 12 months or longer, fair value | 6,034,000 |
Temporarily impaired securities, 12 months or longer, unrealized losses | 195,000 |
Temporarily impaired securities, total fair value | 13,439,000 |
Temporarily impaired securities, total unrealized losses | $ 4,580,000 |

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