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Note 4 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Investment securities available for sale, amortized cost | $ 1,267,731 | $ 1,336,345
Investment securities available for sale, gross unrealized gains | 1,778 | 13,965
Investment securities available for sale, gross unrealized losses | 27,000 | 16,684
Investment securities available for sale, Estimated fair value | 1,242,509 | 1,333,626
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) | 1,242,509 | 1,333,626
US Treasury Securities [Member] |  |
Investment securities available for sale, amortized cost | 124,801 | 249,877
Investment securities available for sale, gross unrealized gains |  |
Investment securities available for sale, gross unrealized losses | 50 | 357
Investment securities available for sale, Estimated fair value | 124,751 | 249,520
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) | 124,751 | 249,520
US Government Corporations and Agencies Securities [Member] |  |
Investment securities available for sale, amortized cost | 6,066 | 9,047
Investment securities available for sale, gross unrealized gains |  | 11
Investment securities available for sale, gross unrealized losses | 195 | 70
Investment securities available for sale, Estimated fair value | 5,871 | 8,988
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) | 5,871 | 8,988
US Government-sponsored Enterprises Debt Securities [Member] |  |
Investment securities available for sale, amortized cost | 400,000 | 400,000
Investment securities available for sale, gross unrealized gains |  |
Investment securities available for sale, gross unrealized losses | 11,638 | 9,664
Investment securities available for sale, Estimated fair value | 388,362 | 390,336
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) | 388,362 | 390,336
Collateralized Mortgage Backed Securities [Member] |  |
Investment securities available for sale, amortized cost | 670,874 | 577,987
Investment securities available for sale, gross unrealized gains | 960 | 241
Investment securities available for sale, gross unrealized losses | 15,089 | 6,259
Investment securities available for sale, Estimated fair value | 656,745 | 571,969
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) | 656,745 | 571,969
US States and Political Subdivisions Debt Securities [Member] |  |
Investment securities available for sale, amortized cost |  | 1,944
Investment securities available for sale, gross unrealized gains |  |
Investment securities available for sale, gross unrealized losses |  | 30
Investment securities available for sale, Estimated fair value |  | 1,914
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) |  | 1,914
Collateralized Mortgage Obligations [Member] |  |
Investment securities available for sale, amortized cost | 1,005 | 1,533
Investment securities available for sale, gross unrealized gains |  |
Investment securities available for sale, gross unrealized losses | 28 | 17
Investment securities available for sale, Estimated fair value | 977 | 1,516
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) | 977 | 1,516
Corporate Debt Securities [Member] |  |
Investment securities available for sale, amortized cost | 64,985 | 80,007
Investment securities available for sale, gross unrealized gains | 818 | 1,291
Investment securities available for sale, gross unrealized losses |  | 17
Investment securities available for sale, Estimated fair value | 65,803 | 81,281
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) | $ 65,803 | 81,281
Mutual Funds [Member] |  |
Investment securities available for sale, amortized cost |  | 6,500
Investment securities available for sale, gross unrealized gains |  |
Investment securities available for sale, gross unrealized losses |  | 270
Investment securities available for sale, Estimated fair value |  | 6,230
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) |  | 6,230
Preferred Stock of Government Sponsored Entities [Member] |  |
Investment securities available for sale, amortized cost |  | 5,842
Investment securities available for sale, gross unrealized gains |  | 4,260
Investment securities available for sale, gross unrealized losses |  |
Investment securities available for sale, Estimated fair value |  | 10,102
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) |  | 10,102
Other Equity Securities [Member] |  |
Investment securities available for sale, amortized cost |  | 3,608
Investment securities available for sale, gross unrealized gains |  | 8,162
Investment securities available for sale, gross unrealized losses |  |
Investment securities available for sale, Estimated fair value |  | 11,770
Securities available-for-sale (amortized cost of $1,267,731 in 2018 and $1,336,345 in 2017) |  | $ 11,770

X
- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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://fasb.org/us-gaap/role/ref/legacyRef -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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