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Note 6 - Credit Quality Indicators (Details Textual) - USD ($)		1 Months Ended	9 Months Ended	12 Months Ended
	Mar. 02, 2026	Mar. 31, 2026	Sep. 30, 2026	Dec. 31, 2025	Jun. 30, 2026	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024
Financing Receivable, Risk Rating Threshold			$ 3,000,000
Other Real Estate			72,000	$ 52,000
Repossessed Assets			1,200,000	506,000
Financing Receivable, before Allowance for Credit Loss			4,854,818,000	3,304,713,000
Financing Receivable, Allowance for Credit Loss			53,531,000	36,811,000	$ 53,285,000	$ 39,528,000	$ 38,563,000	$ 35,863,000
Financing Receivable, Purchased with Credit Deterioration, Fair Value			45,200,000
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date			4,063,000
Financing Receivable, Purchased with Credit Deterioration, Discount (Premium)			5,300,000
Middlefield Banc Corp [Member]
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)			16,700,000
Financing Receivable, Purchase	$ 1,490,000
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date		$ 4,063,000
Financing Receivable, Purchased with Credit Deterioration, Discount (Premium)		$ 21,229,000
Unfunded Loan Commitment [Member]
Financing Receivable, before Allowance for Credit Loss			1,090,000,000.00	710,000,000
Financing Receivable, Allowance for Credit Loss			2,270,000	1,340,000
Unfunded Loan Commitment, Related to Merger [Member]
Increase (Decrease) in Notes Receivables			305,000,000
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)			561,000
Unfunded Loan Commitment, Related to Construction [Member]
Increase (Decrease) in Notes Receivables				77,000,000
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)				$ 372,000
Including Day 1 Reserve Related to Merger [Member]
Financing Receivable, Allowance for Credit Loss			$ 53,500,000