Note 3 - Investment Securities - Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investment securities available for sale, amortized cost	$ 1,019,230	$ 1,443,730
Investment securities available for sale, gross unrealized gains	18,779	9,852
Investment securities available for sale, gross unrealized losses	1,459	1,740
Investment securities available for sale, Estimated fair value	1,036,550	1,451,842
US Treasury Securities [Member]
Investment securities available for sale, amortized cost	80,948	74,926
Investment securities available for sale, gross unrealized gains	6	10
Investment securities available for sale, gross unrealized losses	6	0
Investment securities available for sale, Estimated fair value	80,948	74,936
US Government Corporations and Agencies Securities [Member]
Investment securities available for sale, amortized cost	99,944	90,452
Investment securities available for sale, gross unrealized gains	441	663
Investment securities available for sale, gross unrealized losses	546	319
Investment securities available for sale, Estimated fair value	99,839	90,796
Collateralized Mortgage Backed Securities [Member]
Investment securities available for sale, amortized cost	709,709	880,040
Investment securities available for sale, gross unrealized gains	17,965	8,574
Investment securities available for sale, gross unrealized losses	606	824
Investment securities available for sale, Estimated fair value	727,068	887,790
US Government-sponsored Enterprises Debt Securities [Member]
Investment securities available for sale, amortized cost		225,000
Investment securities available for sale, gross unrealized gains		0
Investment securities available for sale, gross unrealized losses		557
Investment securities available for sale, Estimated fair value		224,443
Collateralized Mortgage Obligations [Member]
Investment securities available for sale, amortized cost	10,358	569
Investment securities available for sale, gross unrealized gains	0	0
Investment securities available for sale, gross unrealized losses	34	17
Investment securities available for sale, Estimated fair value	10,324	552
Corporate Debt Securities [Member]
Investment securities available for sale, amortized cost	118,271	172,743
Investment securities available for sale, gross unrealized gains	367	605
Investment securities available for sale, gross unrealized losses	267	23
Investment securities available for sale, Estimated fair value	$ 118,371	$ 173,325

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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