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Note 3 - Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Held-to-Maturity |  |
Amortized Cost | $ 368,058 | $ 462,096
Gross Unrealized Gains | 5,977 | 5,137
Gross Unrealized Losses | (737) | (1,273)
Estimated Fair Value | 373,298 | 465,960
Available-for-Sale |  |
Amortized Cost | 1,612,324 | 1,181,484
Gross Unrealized Gains | 2,100 | 1,279
Gross Unrealized Losses | (24,907) | (25,409)
Estimated Fair Value | 1,589,517 | 1,157,354
US Government Agencies Debt Securities [Member] |  |
Held-to-Maturity |  |
Amortized Cost | 46,945 | 76,875
Gross Unrealized Gains | 7 | 107
Gross Unrealized Losses | (228) | (182)
Estimated Fair Value | 46,724 | 76,800
Available-for-Sale |  |
Amortized Cost | 141,559 | 140,005
Gross Unrealized Gains | 116 | 67
Gross Unrealized Losses | (1,951) | (2,301)
Estimated Fair Value | 139,724 | 137,771
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |
Held-to-Maturity |  |
Amortized Cost | 16,132 | 19,773
Gross Unrealized Gains | 8 | 63
Gross Unrealized Losses | (287) | (249)
Estimated Fair Value | 15,853 | 19,587
Available-for-Sale |  |
Amortized Cost | 1,208,017 | 885,783
Gross Unrealized Gains | 246 | 178
Gross Unrealized Losses | (20,946) | (17,637)
Estimated Fair Value | 1,187,317 | 868,324
US States and Political Subdivisions Debt Securities [Member] |  |
Held-to-Maturity |  |
Amortized Cost | 301,491 | 362,532
Gross Unrealized Gains | 5,962 | 4,967
Gross Unrealized Losses | (222) | (842)
Estimated Fair Value | 307,231 | 366,657
Available-for-Sale |  |
Amortized Cost | 144,642 | 108,374
Gross Unrealized Gains | 532 | 38
Gross Unrealized Losses | (2,009) | (5,469)
Estimated Fair Value | 143,165 | 102,943
Other Securities [Member] |  |
Held-to-Maturity |  |
Amortized Cost | 3,490 | 2,916
Gross Unrealized Gains |  |
Gross Unrealized Losses |  |
Estimated Fair Value | 3,490 | 2,916
Available-for-Sale |  |
Amortized Cost | 118,106 | 47,022
Gross Unrealized Gains | 1,206 | 996
Gross Unrealized Losses | (1) | (2)
Estimated Fair Value | 119,311 | 48,016
US Treasury Securities [Member] |  |
Available-for-Sale |  |
Amortized Cost |  | 300
Gross Unrealized Gains |  |
Gross Unrealized Losses |  |
Estimated Fair Value |  | $ 300

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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