Note 4 - 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Provision for loan losses	$ 1,750,000	$ 1,100,000	$ 2,950,000
Ending balance	2,856,667,000	2,753,085,000
Originated Loan [Member]
Beginning balance	21,554,000	19,133,000	17,868,000
Provision for loan losses	1,867,000	400,000	2,641,000
Charge-offs	(883,000)	(1,189,000)	(3,183,000)
Recoveries	628,000	3,210,000	1,807,000
Ending balance	23,166,000	21,554,000	19,133,000
Ending balance: individually evaluated for impairment	1,397,000	688,000	1,867,000
Ending balance: collectively evaluated for impairment	21,769,000	20,866,000	17,266,000
Ending balance	2,614,725,000	2,452,446,000	2,169,957,000
Ending balance: individually evaluated for impairment	11,365,000	18,882,000	8,787,000
Ending balance: collectively evaluated for impairment	2,603,360,000	2,433,564,000	2,161,170,000
Acquired Loan [Member]
Beginning balance	826,000	368,000	93,000
Provision for loan losses	(117,000)	700,000	309,000
Charge-offs	0	(261,000)	(52,000)
Recoveries	14,000	19,000	18,000
Ending balance	723,000	826,000	368,000
Ending balance	241,942,000	300,639,000
Commercial Portfolio Segment [Member]
Ending balance	2,441,544,000	2,360,106,000
Commercial Portfolio Segment [Member] | Originated Loan [Member]
Beginning balance	19,442,000	16,456,000	16,026,000
Provision for loan losses	1,521,000	826,000	1,148,000
Charge-offs	(455,000)	(428,000)	(2,292,000)
Recoveries	326,000	2,588,000	1,574,000
Ending balance	20,834,000	19,442,000	16,456,000
Ending balance: individually evaluated for impairment	1,150,000	451,000	1,202,000
Ending balance: collectively evaluated for impairment	19,684,000	18,991,000	15,254,000
Ending balance	2,271,417,000	2,151,998,000	1,934,228,000
Ending balance: individually evaluated for impairment	9,828,000	17,134,000	6,394,000
Ending balance: collectively evaluated for impairment	2,261,589,000	2,134,864,000	1,927,834,000
Commercial Portfolio Segment [Member] | Acquired Loan [Member]
Beginning balance	177,000	291,000	75,000
Provision for loan losses	57,000	132,000	210,000
Charge-offs	0	(246,000)	(12,000)
Recoveries	2,000	0	18,000
Ending balance	236,000	177,000	291,000
Ending balance	170,127,000	208,108,000
Retail Portfolio Segment [Member]
Ending balance	415,123,000	392,979,000
Retail Portfolio Segment [Member] | Originated Loan [Member]
Beginning balance	2,068,000	2,584,000	1,882,000
Provision for loan losses	320,000	(377,000)	1,360,000
Charge-offs	(428,000)	(761,000)	(891,000)
Recoveries	302,000	622,000	233,000
Ending balance	2,262,000	2,068,000	2,584,000
Ending balance: individually evaluated for impairment	247,000	237,000	665,000
Ending balance: collectively evaluated for impairment	2,015,000	1,831,000	1,919,000
Ending balance	343,308,000	300,448,000	235,729,000
Ending balance: individually evaluated for impairment	1,537,000	1,748,000	2,393,000
Ending balance: collectively evaluated for impairment	341,771,000	298,700,000	233,336,000
Retail Portfolio Segment [Member] | Acquired Loan [Member]
Beginning balance	649,000	77,000	18,000
Provision for loan losses	(174,000)	568,000	99,000
Charge-offs	0	(15,000)	(40,000)
Recoveries	12,000	19,000	0
Ending balance	487,000	649,000	77,000
Ending balance	71,815,000	92,531,000
Unallocated Financing Receivables [Member] | Originated Loan [Member]
Beginning balance	44,000	93,000	(40,000)
Provision for loan losses	26,000	(49,000)	133,000
Charge-offs	0	0	0
Recoveries	0	0	0
Ending balance	70,000	44,000	93,000
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	70,000	44,000	93,000
Unallocated Financing Receivables [Member] | Acquired Loan [Member]
Beginning balance	0	0	0
Provision for loan losses	0	0	0
Charge-offs	0	0	0
Recoveries	0	0	0
Ending balance	$ 0	$ 0	$ 0