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, 2018	Dec. 31, 2017	Dec. 31, 2016
Provision for loan losses	$ 1,100,000	$ 2,950,000	$ 2,900,000
Ending balance	2,753,085,000	2,558,552,000
Originated Loan [Member]
Beginning balance	19,133,000	17,868,000	15,233,000
Provision for loan losses	400,000	2,641,000	3,098,000
Charge-offs	(1,189,000)	(3,183,000)	(2,133,000)
Recoveries	3,210,000	1,807,000	1,670,000
Ending balance	21,554,000	19,133,000	17,868,000
Ending balance: individually evaluated for impairment	688,000	1,867,000	1,572,000
Ending balance: collectively evaluated for impairment	20,866,000	17,266,000	16,296,000
Ending balance	2,452,446,000	2,169,957,000	1,884,548,000
Ending balance: individually evaluated for impairment	18,882,000	8,787,000	13,383,000
Ending balance: collectively evaluated for impairment	2,433,564,000	2,161,170,000	1,871,165,000
Acquired Loan [Member]
Beginning balance	368,000	93,000	448,000
Provision for loan losses	700,000	309,000	(198,000)
Charge-offs	(261,000)	(52,000)	(72,000)
Recoveries	19,000	18,000
Recoveries			(85,000)
Ending balance	826,000	368,000	93,000
Ending balance	300,639,000	388,595,000
Commercial Portfolio Segment [Member]
Ending balance	2,360,106,000	2,203,568,000
Commercial Portfolio Segment [Member] | Originated Loan [Member]
Beginning balance	16,456,000	16,026,000	13,672,000
Provision for loan losses	826,000	1,148,000	2,247,000
Charge-offs	(428,000)	(2,292,000)	(980,000)
Recoveries	2,588,000	1,574,000	1,087,000
Ending balance	19,442,000	16,456,000	16,026,000
Ending balance: individually evaluated for impairment	451,000	1,202,000	1,303,000
Ending balance: collectively evaluated for impairment	18,991,000	15,254,000	14,723,000
Ending balance	2,151,998,000	1,934,228,000	1,728,898,000
Ending balance: individually evaluated for impairment	17,134,000	6,394,000	12,070,000
Ending balance: collectively evaluated for impairment	2,134,864,000	1,927,834,000	1,716,828,000
Commercial Portfolio Segment [Member] | Acquired Loan [Member]
Beginning balance	291,000	75,000	420,000
Provision for loan losses	132,000	210,000	(303,000)
Charge-offs	(246,000)	(12,000)	0
Recoveries	0	18,000
Recoveries			(42,000)
Ending balance	177,000	291,000	75,000
Ending balance	208,108,000	269,340,000
Retail Portfolio Segment [Member]
Ending balance	392,979,000	354,984,000
Retail Portfolio Segment [Member] | Originated Loan [Member]
Beginning balance	2,584,000	1,882,000	1,421,000
Provision for loan losses	(377,000)	1,360,000	1,031,000
Charge-offs	(761,000)	(891,000)	(1,153,000)
Recoveries	622,000	233,000	583,000
Ending balance	2,068,000	2,584,000	1,882,000
Ending balance: individually evaluated for impairment	237,000	665,000	269,000
Ending balance: collectively evaluated for impairment	1,831,000	1,919,000	1,613,000
Ending balance	300,448,000	235,729,000	155,650,000
Ending balance: individually evaluated for impairment	1,748,000	2,393,000	1,313,000
Ending balance: collectively evaluated for impairment	298,700,000	233,336,000	154,337,000
Retail Portfolio Segment [Member] | Acquired Loan [Member]
Beginning balance	77,000	18,000	28,000
Provision for loan losses	568,000	99,000	105,000
Charge-offs	(15,000)	(40,000)	(72,000)
Recoveries	19,000	0
Recoveries			(43,000)
Ending balance	649,000	77,000	18,000
Ending balance	92,531,000	119,255,000
Unallocated Financing Receivables [Member] | Originated Loan [Member]
Beginning balance	93,000	(40,000)	140,000
Provision for loan losses	(49,000)	133,000	(180,000)
Charge-offs	0	0	0
Recoveries	0	0	0
Ending balance	44,000	93,000	(40,000)
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	44,000	93,000	(40,000)
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
Recoveries			0
Ending balance	$ 0	$ 0	$ 0