Note 3 - Loans and Allowance for Loan Losses - Activity in Allowance for Loan Losses and the Recorded Investments in Loans (Details) - USD ($)		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
Beginning balance		$ 23,889,000	$ 22,380,000	$ 19,501,000
Provision for loan losses		14,050,000	1,750,000	1,100,000
Charge-offs		(838,000)	(883,000)	(1,450,000)
Recoveries		866,000	642,000	3,229,000
Ending balance		37,967,000	23,889,000	22,380,000
Ending balance: individually evaluated for impairment					$ 551,000		$ 1,689,000	$ 1,089,000
Ending balance: collectively evaluated for impairment					37,416,000		22,200,000	21,291,000
Ending balance	[1]				2,851,099,000
Ending balance: individually evaluated for impairment					26,958,000	[1]	14,622,000	23,908,000
Ending balance: collectively evaluated for impairment					2,824,141,000	[1]	2,842,045,000	2,729,177,000
Loans and Leases Receivable, Net of Deferred Income, Total		3,216,358,000	2,856,667,000	2,753,085,000	3,216,358,000		2,856,667,000	2,753,085,000
Ending balance		3,216,358,000	2,856,667,000	2,753,085,000
Commercial Portfolio Segment [Member]
Beginning balance		21,070,000	19,619,000	16,747,000
Provision for loan losses		12,835,000	1,578,000	958,000
Charge-offs		(613,000)	(455,000)	(674,000)
Recoveries		487,000	328,000	2,588,000
Ending balance		33,779,000	21,070,000	19,619,000
Ending balance: individually evaluated for impairment					138,000		1,230,000	550,000
Ending balance: collectively evaluated for impairment					33,641,000		19,840,000	19,069,000
Ending balance	[1]				2,428,703,000
Ending balance: individually evaluated for impairment					22,415,000	[1]	10,604,000	19,291,000
Ending balance: collectively evaluated for impairment					2,406,288,000	[1]	2,430,940,000	2,340,815,000
Loans and Leases Receivable, Net of Deferred Income, Total		2,793,962,000	2,441,544,000	2,360,106,000	2,793,962,000		2,441,544,000	2,360,106,000
Ending balance		2,793,962,000	2,441,544,000	2,360,106,000
Retail Portfolio Segment [Member]
Beginning balance		2,749,000	2,717,000	2,661,000
Provision for loan losses		1,226,000	146,000	191,000
Charge-offs		(225,000)	(428,000)	(776,000)
Recoveries		379,000	314,000	641,000
Ending balance		4,129,000	2,749,000	2,717,000
Ending balance: individually evaluated for impairment					413,000		459,000	539,000
Ending balance: collectively evaluated for impairment					3,716,000		2,290,000	2,178,000
Ending balance	[1]				422,396,000
Ending balance: individually evaluated for impairment					4,543,000	[1]	4,018,000	4,617,000
Ending balance: collectively evaluated for impairment					417,853,000	[1]	411,105,000	388,362,000
Loans and Leases Receivable, Net of Deferred Income, Total		422,396,000	415,123,000	392,979,000	422,396,000		415,123,000	392,979,000
Ending balance		422,396,000	415,123,000	392,979,000
Unallocated Financing Receivables [Member]
Beginning balance		70,000	44,000	93,000
Provision for loan losses		(11,000)	26,000	(49,000)
Charge-offs		0	0	0
Recoveries		0	0	0
Ending balance		$ 59,000	$ 70,000	$ 44,000
Ending balance: individually evaluated for impairment					0		0	0
Ending balance: collectively evaluated for impairment					$ 59,000		$ 70,000	$ 44,000

[1] Excludes $365 million in loans originated under the Paycheck Protection Program.