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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Provision for loan losses	$ 400,000	$ 1,000,000	$ 1,100,000	$ 2,350,000
Ending balance	2,697,417,000		2,697,417,000		$ 2,558,552,000
Originated Loan [Member]
Beginning balance	20,482,000	17,955,000	19,133,000	17,868,000
Provision for loan losses	473,000	913,000	605,000	2,023,000
Charge-offs	(169,000)	(667,000)	(835,000)	(2,262,000)
Recoveries	290,000	606,000	2,173,000	1,178,000
Ending balance	21,076,000	18,807,000	21,076,000	18,807,000
Ending balance: individually evaluated for impairment	998,000	2,594,000	998,000	2,594,000
Ending balance: collectively evaluated for impairment	20,078,000	16,213,000	20,078,000	16,213,000
Ending balance	2,383,104,000		2,383,104,000		2,169,957,000
Ending balance: individually evaluated for impairment	11,945,000		11,945,000		8,787,000
Ending balance: collectively evaluated for impairment	2,371,159,000		2,371,159,000		2,161,170,000
Acquired Loan [Member]
Beginning balance	685,000	340,000	368,000	93,000
Provision for loan losses	(73,000)	87,000	495,000	327,000
Charge-offs	0	(42,000)	(261,000)	(53,000)
Recoveries	4,000	1,000	14,000	19,000
Ending balance	616,000	386,000	616,000	386,000
Ending balance	314,313,000		314,313,000		388,595,000
Commercial Portfolio Segment [Member]
Ending balance	2,306,107,000		2,306,107,000		2,203,568,000
Commercial Portfolio Segment [Member] | Originated Loan [Member]
Beginning balance	18,326,000	15,363,000	16,456,000	16,026,000
Provision for loan losses	371,000	452,000	922,000	600,000
Charge-offs	0	(347,000)	(342,000)	(1,579,000)
Recoveries	63,000	565,000	1,724,000	986,000
Ending balance	18,760,000	16,033,000	18,760,000	16,033,000
Ending balance: individually evaluated for impairment	750,000	1,856,000	750,000	1,856,000
Ending balance: collectively evaluated for impairment	18,010,000	14,177,000	18,010,000	14,177,000
Ending balance	2,090,476,000		2,090,476,000		1,934,228,000
Ending balance: individually evaluated for impairment	9,929,000		9,929,000		6,394,000
Ending balance: collectively evaluated for impairment	2,080,547,000		2,080,547,000		1,927,834,000
Commercial Portfolio Segment [Member] | Acquired Loan [Member]
Beginning balance	105,000	324,000	291,000	75,000
Provision for loan losses	9,000	(18,000)	69,000	224,000
Charge-offs	0	(1,000)	(246,000)	(12,000)
Recoveries	0	1,000	0	19,000
Ending balance	114,000	306,000	114,000	306,000
Ending balance	215,631,000		215,631,000		269,340,000
Retail Portfolio Segment [Member]
Ending balance	391,310,000		391,310,000		354,984,000
Retail Portfolio Segment [Member] | Originated Loan [Member]
Beginning balance	2,216,000	2,433,000	2,584,000	1,882,000
Provision for loan losses	(41,000)	415,000	(307,000)	1,178,000
Charge-offs	(169,000)	(320,000)	(493,000)	(683,000)
Recoveries	227,000	41,000	449,000	192,000
Ending balance	2,233,000	2,569,000	2,233,000	2,569,000
Ending balance: individually evaluated for impairment	248,000	738,000	248,000	738,000
Ending balance: collectively evaluated for impairment	1,985,000	1,831,000	1,985,000	1,831,000
Ending balance	292,628,000		292,628,000		235,729,000
Ending balance: individually evaluated for impairment	2,016,000		2,016,000		2,393,000
Ending balance: collectively evaluated for impairment	290,612,000		290,612,000		233,336,000
Retail Portfolio Segment [Member] | Acquired Loan [Member]
Beginning balance	580,000	16,000	77,000	18,000
Provision for loan losses	(82,000)	105,000	426,000	103,000
Charge-offs	0	(41,000)	(15,000)	(41,000)
Recoveries	4,000	0	14,000	0
Ending balance	502,000	80,000	502,000	80,000
Ending balance	98,682,000		98,682,000		$ 119,255,000
Unallocated Financing Receivables [Member] | Originated Loan [Member]
Beginning balance	(60,000)	159,000	93,000	(40,000)
Provision for loan losses	143,000	46,000	(10,000)	245,000
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending balance	83,000	205,000	83,000	205,000
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	83,000	205,000	83,000	205,000
Unallocated Financing Receivables [Member] | Acquired Loan [Member]
Beginning balance	0	0	0	0
Provision for loan losses	0	0	0	0
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending balance	$ 0	$ 0	$ 0	$ 0