Note 3 - Investment Securities - Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Securities held to maturity:
Securities held to maturity, due in one year or less, amortized cost	$ 3,417
Securities held to maturity, due in one year or less, fair value	3,443
Securities held to maturity, due after one year through five years, amortized cost	34,396
Securities held to maturity, due after one year through five years, fair value	35,103
Securities held to maturity, due after five years, amortized cost	438,352
Securities held to maturity, due after five years, fair value	482,731
Securities held to maturity, amortized cost	476,165
Securities held to maturity, fair value	521,277	$ 426,545
Securities available for sale:
Securities available for sale, due in one year or less, amortized cost	2,369
Securities available for sale, Due in one year or less, fair value	2,373
Securities available for sale, due after one year through five years, amortized cost	13,970
Securities available for sale, Due after one year through five years, fair value	14,328
Securities available for sale, due after five years, amortized cost	166,388
Securities available for sale, Due after five years, fair value	171,740
Securities available for sale, single maturity, amortized cost	182,727
Securities available for sale, single maturity, fair value	188,441
Securities available for sale, amortized cost	350,060	206,962
Securities available for sale, fair value	361,966	210,695
Callable Securities [Member]
Securities available for sale:
Securities available for sale, amortized cost	145,188
Securities available for sale, fair value	150,183
Municipal securities
Securities held to maturity:
Securities held to maturity, fair value	520,227	425,495
Securities available for sale:
Securities available for sale, amortized cost	147,241	46,659
Securities available for sale, fair value	152,408	48,257
Municipal securities | Callable Securities [Member]
Securities held to maturity:
Securities held to maturity, callable, amortized cost	227,773
Securities held to maturity, callable, fair value	236,134
Securities available for sale:
Securities available for sale, callable, amortized cost	138,688
Securities available for sale, callable, fair value	143,536
Asset-backed securities
Securities available for sale:
Securities available for sale, amortized cost	39,663	16,958
Securities available for sale, fair value	40,683	16,887
Securities available for sale, callable, amortized cost	39,663
Securities available for sale, callable, fair value	40,683
Residential mortgage-backed and related securities
Securities available for sale:
Securities available for sale, amortized cost	127,670	118,724
Securities available for sale, fair value	132,842	120,587
Securities available for sale, callable, amortized cost	127,670
Securities available for sale, callable, fair value	132,842
Other Securities
Securities held to maturity:
Securities held to maturity, fair value	1,050	1,050
Securities available for sale:
Securities available for sale, amortized cost	20,550	4,749
Securities available for sale, fair value	20,697	$ 4,886
Securities available for sale, callable, amortized cost	6,500
Securities available for sale, callable, fair value	$ 6,647