Note 8 - Loans - Allowance for Loan Losses by Portfolio Segment and Based on Impairment Method (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Loans individually evaluated for impairment, allowance	$ 3,704		$ 3,704		$ 2,134
Loans individually evaluated for impairment, balance	117,010		117,010		117,353
Loans collectively evaluated for impairment, allowance	119,753		119,753		121,145
Loans collectively evaluated for impairment, balance	13,530,636		13,530,636		12,752,937
Loans, allowance	123,457		123,457		123,279
Loans	13,647,646		13,647,646		12,870,290
Beginning Balance	121,899	$ 115,809	123,279	$ 118,966
Provision/(credit) for possible credit losses	(1,500)		(4,500)	(2,500)
Charge-offs	(122)	(385)	(1,019)	(2,670)
Recoveries	3,180	6,111	5,697	7,739
Net (charge-offs)/recoveries	3,058	5,726	4,678	5,069
Ending Balance	123,457	121,535	123,457	121,535
Reserve for impaired loans	3,704	2,606	3,704	2,606
Reserve for non-impaired loans	119,753	118,929	119,753	118,929
Reserve for off-balance sheet credit commitments	3,088	4,254	3,088	4,254
Commercial Portfolio Segment [Member]
Loans individually evaluated for impairment, allowance	2,506		2,506		43
Loans individually evaluated for impairment, balance	44,272		44,272		43,495
Loans collectively evaluated for impairment, allowance	54,008		54,008		49,753
Loans collectively evaluated for impairment, balance	2,629,817		2,629,817		2,417,771
Loans, allowance	56,514		56,514		49,796
Loans	2,674,089		2,674,089		2,461,266
Beginning Balance	55,179	46,744	49,796	49,203
Provision/(credit) for possible credit losses	1,270	3,800	6,097	2,245
Charge-offs	(122)	(80)	(629)	(1,810)
Recoveries	187	575	1,250	1,401
Net (charge-offs)/recoveries	65	495	621	(409)
Ending Balance	56,514	51,039	56,514	51,039
Reserve for impaired loans	2,506	1,461	2,506	1,461
Reserve for non-impaired loans	54,008	49,578	54,008	49,578
Reserve for off-balance sheet credit commitments	1,615	2,760	1,615	2,760
Real Estate Construction Portfolio Segment [Member]
Loans individually evaluated for impairment, allowance
Loans individually evaluated for impairment, balance	4,922		4,922		8,185
Loans collectively evaluated for impairment, allowance	21,226		21,226		24,838
Loans collectively evaluated for impairment, balance	592,096		592,096		670,620
Loans, allowance	21,226		21,226		24,838
Loans	597,018		597,018		678,805
Beginning Balance	20,663	17,844	24,838	23,268
Provision/(credit) for possible credit losses	519	4,117	(3,744)	(1,403)
Charge-offs
Recoveries	44	47	132	143
Net (charge-offs)/recoveries	44	47	132	143
Ending Balance	21,226	22,008	21,226	22,008
Reserve for impaired loans
Reserve for non-impaired loans	21,226	22,008	21,226	22,008
Reserve for off-balance sheet credit commitments	1,174	1,206	1,174	1,206
Commercial Real Estate Portfolio Segment [Member]
Loans individually evaluated for impairment, allowance	917		917		1,738
Loans individually evaluated for impairment, balance	54,435		54,435		52,664
Loans collectively evaluated for impairment, allowance	31,865		31,865		35,872
Loans collectively evaluated for impairment, balance	6,525,819		6,525,819		6,430,031
Loans, allowance	32,782		32,782		37,610
Loans	6,580,254		6,580,254		6,482,695
Beginning Balance	33,976	36,840	37,610	34,864
Provision/(credit) for possible credit losses	(4,138)	(4,615)	(8,672)	(2,775)
Charge-offs		(305)	(390)	(860)
Recoveries	2,944	5,482	4,234	6,173
Net (charge-offs)/recoveries	2,944	5,177	3,844	5,313
Ending Balance	32,782	37,402	32,782	37,402
Reserve for impaired loans	917	823	917	823
Reserve for non-impaired loans	31,865	36,579	31,865	36,579
Reserve for off-balance sheet credit commitments	78	109	78	109
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans individually evaluated for impairment, allowance	281		281		353
Loans individually evaluated for impairment, balance	13,381		13,381		13,009
Loans collectively evaluated for impairment, allowance	12,628		12,628		10,660
Loans collectively evaluated for impairment, balance	3,777,329		3,777,329		3,229,345
Loans, allowance	12,909		12,909		11,013
Loans	3,790,710		3,790,710		3,242,354
Beginning Balance	12,062	14,364	11,013	11,620
Provision/(credit) for possible credit losses	842	(3,309)	1,815	(580)
Charge-offs
Recoveries	5	7	81	22
Net (charge-offs)/recoveries	5	7	81	22
Ending Balance	12,909	11,062	12,909	11,062
Reserve for impaired loans	281	322	281	322
Reserve for non-impaired loans	12,628	10,740	12,628	10,740
Reserve for off-balance sheet credit commitments	215	175	215	175
Installment and Other Loans [Member ]
Loans	5,575		5,575		5,170
Beginning Balance	19	17	22	11
Provision/(credit) for possible credit losses	7	7	4	13
Charge-offs
Recoveries
Net (charge-offs)/recoveries
Ending Balance	26	24	26	24
Reserve for impaired loans
Reserve for non-impaired loans	26	24	26	24
Reserve for off-balance sheet credit commitments	6	$ 4	6	$ 4
Consumer Portfolio Segment [Member]
Loans individually evaluated for impairment, allowance
Loans individually evaluated for impairment, balance
Loans collectively evaluated for impairment, allowance	26		26		22
Loans collectively evaluated for impairment, balance	5,575		5,575		5,170
Loans, allowance	26		26		22
Loans	$ 5,575		$ 5,575		$ 5,170