Note 3 - Investment Securities - Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Securities held to maturity:
Securities held to maturity, due in one year or less, amortized cost	$ 3,220
Securities held to maturity, due in one year or less, fair value	3,234
Securities held to maturity, due after one year through five years, amortized cost	33,088
Securities held to maturity, due after one year through five years, fair value	33,865
Securities held to maturity, due after five years, amortized cost	364,338
Securities held to maturity, due after five years, fair value	389,446
Securities held to maturity, amortized cost	400,646
Securities held to maturity, fair value	426,545	$ 400,770
Securities available for sale:
Securities available for sale, due in one year or less, amortized cost	1,084
Securities available for sale, Due in one year or less, fair value	1,084
Securities available for sale, due after one year through five years, amortized cost	17,089
Securities available for sale, Due after one year through five years, fair value	17,320
Securities available for sale, due after five years, amortized cost	70,065
Securities available for sale, Due after five years, fair value	71,704
Securities available for sale, single maturity, amortized cost	88,238
Securities available for sale, single maturity, fair value	90,108
Securities available for sale, amortized cost	206,962	266,671
Securities available for sale, fair value	210,695	261,056
Callable Securities [Member]
Securities available for sale:
Securities available for sale, amortized cost	44,174
Securities available for sale, fair value	45,628
US States and Political Subdivisions Debt Securities
Securities held to maturity:
Securities held to maturity, fair value	425,495	399,721
Securities available for sale:
Securities available for sale, amortized cost	46,659	59,069
Securities available for sale, fair value	48,257	58,546
US States and Political Subdivisions Debt Securities | Callable Securities [Member]
Securities held to maturity:
Securities held to maturity, callable, amortized cost	182,653
Securities held to maturity, callable, fair value	186,631
Securities available for sale:
Securities available for sale, callable, amortized cost	39,674
Securities available for sale, callable, fair value	40,990
Residential mortgage-backed and related securities
Securities available for sale:
Securities available for sale, amortized cost	118,724	163,698
Securities available for sale, fair value	120,587	159,249
Securities available for sale, callable, amortized cost	118,724
Securities available for sale, callable, fair value	120,587
Other Securities
Securities held to maturity:
Securities held to maturity, fair value	1,050	1,049
Securities available for sale:
Securities available for sale, amortized cost	21,707	6,754
Securities available for sale, fair value	21,773	$ 6,850
Securities available for sale, callable, amortized cost	4,500
Securities available for sale, callable, fair value	$ 4,638