Note 3 - Loans and Allowance for Loan Losses - Activity in Allowance for Loan Losses and the Recorded Investments in Loans (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Provision for loan losses	$ 0	$ 600,000
Loans and Leases Receivable, Net of Deferred Income, Total	2,551,204,000		$ 2,558,552,000
Originated Loan [Member]
Beginning balance	19,133,000	17,868,000	17,868,000
Provision for loan losses	(782,000)		2,641,000
Charge-offs	(393,000)		(3,183,000)
Recoveries	1,123,000		1,807,000
Ending balance	19,081,000		19,133,000
Ending balance: individually evaluated for impairment	863,000		1,867,000
Ending balance: collectively evaluated for impairment	18,218,000		17,266,000
Loans and Leases Receivable, Net of Deferred Income, Total	2,191,851,000		2,169,957,000
Ending balance: individually evaluated for impairment	9,420,000		8,787,000
Ending balance: collectively evaluated for impairment	2,182,431,000		2,161,170,000
Acquired Loan [Member]
Beginning balance	368,000	93,000	93,000
Provision for loan losses	782,000		309,000
Charge-offs	(261,000)		(52,000)
Recoveries	4,000		18,000
Ending balance	893,000		368,000
Loans and Leases Receivable, Net of Deferred Income, Total	359,353,000		388,595,000
Commercial Portfolio Segment [Member]
Loans and Leases Receivable, Net of Deferred Income, Total	2,193,029,000		2,203,568,000
Commercial Portfolio Segment [Member] | Originated Loan [Member]
Beginning balance	16,456,000	16,026,000	16,026,000
Provision for loan losses	(341,000)		1,148,000
Charge-offs	(258,000)		(2,292,000)
Recoveries	1,008,000		1,574,000
Ending balance	16,865,000		16,456,000
Ending balance: individually evaluated for impairment	547,000		1,202,000
Ending balance: collectively evaluated for impairment	16,318,000		15,254,000
Loans and Leases Receivable, Net of Deferred Income, Total	1,946,966,000		1,934,228,000
Ending balance: individually evaluated for impairment	7,398,000		6,394,000
Ending balance: collectively evaluated for impairment	1,939,568,000		1,927,834,000
Commercial Portfolio Segment [Member] | Acquired Loan [Member]
Beginning balance	291,000	75,000	75,000
Provision for loan losses	314,000		210,000
Charge-offs	(246,000)		(12,000)
Recoveries	0		18,000
Ending balance	359,000		291,000
Loans and Leases Receivable, Net of Deferred Income, Total	246,063,000		269,340,000
Retail Portfolio Segment [Member]
Loans and Leases Receivable, Net of Deferred Income, Total	358,175,000		354,984,000
Retail Portfolio Segment [Member] | Originated Loan [Member]
Beginning balance	2,584,000	1,882,000	1,882,000
Provision for loan losses	(563,000)		1,360,000
Charge-offs	(135,000)		(891,000)
Recoveries	115,000		233,000
Ending balance	2,001,000		2,584,000
Ending balance: individually evaluated for impairment	316,000		665,000
Ending balance: collectively evaluated for impairment	1,685,000		1,919,000
Loans and Leases Receivable, Net of Deferred Income, Total	244,885,000		235,729,000
Ending balance: individually evaluated for impairment	2,022,000		2,393,000
Ending balance: collectively evaluated for impairment	242,863,000		233,336,000
Retail Portfolio Segment [Member] | Acquired Loan [Member]
Beginning balance	77,000	18,000	18,000
Provision for loan losses	468,000		99,000
Charge-offs	(15,000)		(40,000)
Recoveries	4,000		0
Ending balance	534,000		77,000
Loans and Leases Receivable, Net of Deferred Income, Total	113,290,000		119,255,000
Unallocated Financing Receivables [Member] | Originated Loan [Member]
Beginning balance	93,000	(40,000)	(40,000)
Provision for loan losses	122,000		133,000
Charge-offs	0		0
Recoveries	0		0
Ending balance	215,000		93,000
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	215,000		93,000
Loans and Leases Receivable, Net of Deferred Income, Total
Ending balance: individually evaluated for impairment
Ending balance: collectively evaluated for impairment
Unallocated Financing Receivables [Member] | Acquired Loan [Member]
Beginning balance	0	$ 0	0
Provision for loan losses	0		0
Charge-offs	0		0
Recoveries	0		0
Ending balance	$ 0		$ 0