Note 8 - Loans (Details Textual) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Impaired Financing Receivable, Recorded Investment, Total	$ 105,778			$ 105,778		$ 75,859
Percentage Of Charge Off To Contractual Balances For Impaired Loans	6.10%			6.10%		2.10%
Financing Receivables, Impaired, Troubled Debt Restructuring, Specific Reserve	$ 203	$ 89		$ 203	$ 125
Financing Receivable, Troubled Debt Restructuring, Subsequent Default				0
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend						$ 0
Loans and Leases Receivable, Gross, Total	15,565,779			15,565,779		15,075,481
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable, Gross, Total	$ 7,459,316			$ 7,459,316		7,275,262
Financing Receivable, Number of Contracts with Modifications	95			95
Loans and Leases Receivable with Modifications, Gross	$ 428,100			$ 428,100
Financing Receivable with Modifications, Percentage of Total Commercial Real Estate Loans	5.70%			5.70%
Commercial Portfolio Segment [Member]
Financing Receivables, Impaired, Troubled Debt Restructuring, Specific Reserve	$ 203	$ 89		$ 203	$ 125
Loans and Leases Receivable, Gross, Total	$ 2,848,000			$ 2,848,000		2,778,744
Financing Receivable, Number of Contracts with Modifications	24			24
Loans and Leases Receivable with Modifications, Gross	$ 64,600			$ 64,600
Financing Receivable, Percentage of Total Modified Commercial Loans	2.30%			2.30%
Residential Portfolio Segment [Member]
Loans and Leases Receivable, Gross, Total	$ 4,169,847			$ 4,169,847		4,088,586
Financing Receivable, Number of Contracts with Approved Forbearance Requests	367			367
Loans and Leases Receivable with Approved Forbearance Requests, Gross	$ 180,600			$ 180,600
Financing Receivable, Approved Forbearance Percentage of Total Residential Mortgage Loans	4.30%			4.30%
HELOC Loans [Member]
Financing Receivable, Number of Contracts with Modifications	21			21
Loans and Leases Receivable with Modifications, Gross	$ 6,500			$ 6,500
Financing Receivable with Modifications, Percentage of Total HELOC Loans	1.60%			1.60%
SBA's Paycheck Protection Program [Member]
Financing Receivable, Number of Contracts	1,437			1,437
Loans and Leases Receivable, Gross, Total	$ 265,700			$ 265,700
Minimum [Member]
Period Loan Is In Payment Default (Day)			60 days
Maximum [Member]
Period Loan Is In Payment Default (Day)			90 days
Nonaccrual Loans [Member]
Impaired Financing Receivable, Recorded Investment, Total	77,200			77,200		40,500
Accruing Troubled Debt Restructuring [Member]
Impaired Financing Receivable, Recorded Investment, Total	28,600			28,600		35,400
Financing Receivable, Troubled Debt Restructuring	28,600			28,600		35,300
Troubled Debt Restructuring, Specific Reserves	395,000			395,000
Financing Receivables, Impaired, Troubled Debt Restructuring, Specific Reserve				822
Non-accruing Troubled Debt Restructuring [Member]
Financing Receivable, Troubled Debt Restructuring	9,900			9,900		18,000
Troubled Debt Restructuring, Specific Reserves	$ 42			$ 42
Financing Receivables, Impaired, Troubled Debt Restructuring, Specific Reserve						$ 2,200