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Note 7 - Investment Securities - Temporarily Impaired Securities (Details) - USD ($) | Jun. 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, unrealized losses |  | 13,341,000
Temporarily impaired securities, total fair value |  | 1,181,295,000
Temporarily impaired securities, total unrealized losses | $ 35,200,000 | 16,684,000
Temporarily impaired securities, 12 months or longer, fair value |  | 624,880,000
US Treasury Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 124,463,000 | 199,823,000
Temporarily impaired securities, less than 12 months, unrealized losses | 49,000 | 62,000
Temporarily impaired securities, 12 months or longer, unrealized losses | 92,000 | 295,000
Temporarily impaired securities, total fair value | 174,369,000 | 249,520,000
Temporarily impaired securities, total unrealized losses | 141,000 | 357,000
Temporarily impaired securities, 12 months or longer, fair value |  | 49,697,000
US Government Corporations and Agencies Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 8,189,000 | 5,711,000
Temporarily impaired securities, less than 12 months, unrealized losses | 208,000 | 70,000
Temporarily impaired securities, 12 months or longer, unrealized losses |  |
Temporarily impaired securities, total fair value | 8,189,000 | 5,711,000
Temporarily impaired securities, total unrealized losses | 208,000 | 70,000
Temporarily impaired securities, 12 months or longer, fair value |  |
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, unrealized losses | 16,041,000 | 9,664,000
Temporarily impaired securities, total fair value | 383,959,000 | 390,336,000
Temporarily impaired securities, total unrealized losses | 16,041,000 | 9,664,000
Temporarily impaired securities, 12 months or longer, fair value | 383,959,000 | 390,336,000
Collateralized Mortgage Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 465,499,000 | 342,436,000
Temporarily impaired securities, less than 12 months, unrealized losses | 12,349,000 | 3,147,000
Temporarily impaired securities, 12 months or longer, unrealized losses | 6,407,000 | 3,112,000
Temporarily impaired securities, total fair value | 625,984,000 | 521,053,000
Temporarily impaired securities, total unrealized losses | 18,756,000 | 6,259,000
Temporarily impaired securities, 12 months or longer, fair value | 160,485,000 | 178,617,000
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, unrealized losses |  |
Temporarily impaired securities, total fair value |  | 1,914,000
Temporarily impaired securities, total unrealized losses |  | 30,000
Temporarily impaired securities, 12 months or longer, fair value |  |
Collateralized Mortgage Obligations [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 1,203,000 | 1,516,000
Temporarily impaired securities, less than 12 months, unrealized losses | 30,000 | 17,000
Temporarily impaired securities, 12 months or longer, unrealized losses |  |
Temporarily impaired securities, total fair value | 1,203,000 | 1,516,000
Temporarily impaired securities, total unrealized losses | 30,000 | 17,000
Temporarily impaired securities, 12 months or longer, fair value |  |
Total Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 599,354,000 |
Temporarily impaired securities, less than 12 months, unrealized losses | 12,636,000 |
Temporarily impaired securities, 12 months or longer, unrealized losses | 22,540,000 |
Temporarily impaired securities, total fair value | 1,193,704,000 |
Temporarily impaired securities, total unrealized losses | 35,176,000 |
Temporarily impaired securities, 12 months or longer, fair value | 594,350,000 |
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, unrealized losses |  |
Temporarily impaired securities, total fair value |  | 5,015,000
Temporarily impaired securities, total unrealized losses |  | 17,000
Temporarily impaired securities, 12 months or longer, fair value |  |
Preferred Stock of Government Sponsored Entities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 7,480,000 |
Temporarily impaired securities, less than 12 months, unrealized losses | 2,623,000 |
Temporarily impaired securities, 12 months or longer, unrealized losses |  |
Temporarily impaired securities, total fair value | 7,480,000 |
Temporarily impaired securities, total unrealized losses | 2,623,000 |
Temporarily impaired securities, 12 months or longer, fair value |  |
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, unrealized losses | 148,000 | 270,000
Temporarily impaired securities, total fair value | 6,082,000 | 6,230,000
Temporarily impaired securities, total unrealized losses | 148,000 | 270,000
Temporarily impaired securities, 12 months or longer, fair value | 6,082,000 | $ 6,230,000
Other Equity Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 9,569,000 |
Temporarily impaired securities, less than 12 months, unrealized losses | 2,200,000 |
Temporarily impaired securities, 12 months or longer, unrealized losses |  |
Temporarily impaired securities, total fair value | 9,569,000 |
Temporarily impaired securities, total unrealized losses | 2,200,000 |
Temporarily impaired securities, 12 months or longer, fair value |  |
Total Equity Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 17,049,000 |
Temporarily impaired securities, less than 12 months, unrealized losses | 4,823,000 |
Temporarily impaired securities, 12 months or longer, unrealized losses | 148,000 |
Temporarily impaired securities, total fair value | 23,131,000 |
Temporarily impaired securities, total unrealized losses | 4,971,000 |
Temporarily impaired securities, 12 months or longer, fair value | $ 6,082,000 |

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