Note 3 - Loans and Allowance for Credit Losses on Loans (Details Textual) Pure in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)	Mar. 31, 2022 USD ($)
Financing Receivable, Accrued Interest, after Allowance for Credit Loss		$ 17,000,000		$ 17,000,000		$ 17,000,000
Financing Receivable, before Allowance for Credit Loss	[1]	5,418,609,000		5,418,609,000		5,205,918,000
Loans and Leases Receivable, Related Parties		74,000,000		74,000,000		79,000,000
Financing Receivable, after Allowance for Credit Loss		5,340,899,000		5,340,899,000		5,132,387,000
Financing Receivable, Purchased with Credit Deterioration, Interest Income		1,100,000	$ 2,000,000.0	2,200,000	$ 2,900,000
Financing Receivable, Nonaccrual, Interest Income		0	$ 0	0	$ 0
Mortgage Loans in Process of Foreclosure, Amount		$ 252,000		$ 252,000			$ 317,000
Financing Receivable, Modifications, Number of Contracts		0	0	0	0
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts		0	0	0	0
Commonwealth [Member] | Financial Asset Acquired with Credit Deterioration [Member]
Financing Receivable, after Allowance for Credit Loss		$ 56,000,000		$ 56,000,000
Asset Pledged as Collateral [Member] | Federal Home Loan Bank Advances [Member]
Financing Receivable, before Allowance for Credit Loss		$ 2,960,000,000		$ 2,960,000,000		$ 2,770,000,000

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