Note 7 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investment securities available for sale, amortized cost	$ 898,463	$ 1,019,230
Investment securities available for sale, gross unrealized gains	16,534	18,779
Investment securities available for sale, gross unrealized losses	6,153	1,459
Investment securities available for sale, Estimated fair value	908,844	1,036,550
US Treasury Securities [Member]
Investment securities available for sale, amortized cost	40,709	80,948
Investment securities available for sale, gross unrealized gains	11	6
Investment securities available for sale, gross unrealized losses	0	6
Investment securities available for sale, Estimated fair value	40,720	80,948
US Government Corporations and Agencies Securities [Member]
Investment securities available for sale, amortized cost	95,637	99,944
Investment securities available for sale, gross unrealized gains	705	441
Investment securities available for sale, gross unrealized losses	291	546
Investment securities available for sale, Estimated fair value	96,051	99,839
Collateralized Mortgage Backed Securities [Member]
Investment securities available for sale, amortized cost	658,371	709,709
Investment securities available for sale, gross unrealized gains	15,292	17,965
Investment securities available for sale, gross unrealized losses	5,239	606
Investment securities available for sale, Estimated fair value	668,424	727,068
Collateralized Mortgage Obligations [Member]
Investment securities available for sale, amortized cost	10,201	10,358
Investment securities available for sale, gross unrealized gains	0	0
Investment securities available for sale, gross unrealized losses	74	34
Investment securities available for sale, Estimated fair value	10,127	10,324
Corporate Debt Securities [Member]
Investment securities available for sale, amortized cost	93,545	118,271
Investment securities available for sale, gross unrealized gains	526	367
Investment securities available for sale, gross unrealized losses	549	267
Investment securities available for sale, Estimated fair value	$ 93,522	$ 118,371

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).

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Amount, before tax, of unrealized loss 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).

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