Note 4 - Loans and Allowance for Credit Losses on Loans (Details Textual) Pure in Thousands		3 Months Ended
		Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)
Financing Receivable, Accrued Interest, after Allowance for Credit Loss		$ 16,000,000		$ 17,000,000
Financing Receivable, before Allowance for Credit Loss	[1]	5,243,104,000		5,205,918,000
Loans and Leases Receivable, Related Parties		71,000,000		79,000,000
Financing Receivable, after Allowance for Credit Loss		5,167,431,000		5,132,387,000
Financing Receivable, Purchased with Credit Deterioration, Interest Income		1,100,000	$ 459,000
Financing Receivable, Nonaccrual, Interest Income		0	0
Mortgage Loans in Process of Foreclosure, Amount		$ 290,000	$ 317,000
Financing Receivable, Modifications, Number of Contracts		0	0
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts		0	0
Commonwealth [Member] | Financial Asset Acquired with Credit Deterioration [Member]
Financing Receivable, after Allowance for Credit Loss		$ 59,000,000
Asset Pledged as Collateral [Member] | Federal Home Loan Bank Advances [Member]
Financing Receivable, before Allowance for Credit Loss		$ 2,850,000,000		$ 2,770,000,000

[1] Total loans are presented inclusive of premiums, discounts and net loan origination fees and costs.