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Note 6 - Credit Quality Indicators (Details Textual) - USD ($)			6 Months Ended
	Jun. 30, 2026	Mar. 02, 2026	Jun. 30, 2026	Mar. 31, 2026	Dec. 31, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024
Financing Receivable, Risk Rating Threshold	$ 3,000,000		$ 3,000,000
Other Real Estate	72,000		72,000		$ 52,000
Repossessed Assets	1,480,000		1,480,000		506,000
Financing Receivable, before Allowance for Credit Loss	4,776,477,000		4,776,477,000		3,304,713,000
Financing Receivable, Allowance for Credit Loss	53,285,000		53,285,000	$ 54,684,000	36,811,000	$ 38,563,000	$ 35,549,000	$ 35,863,000
Financing Receivable, Purchased with Credit Deterioration, Fair Value			41,200,000
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date			3,995,000
Financing Receivable, Purchased with Credit Deterioration, Discount (Premium)			5,300,000
Middlefield Banc Corp [Member]
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)			16,500,000
Financing Receivable, Purchase		$ 1,490,000
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date			3,995,000
Financing Receivable, Purchased with Credit Deterioration, Discount (Premium)			21,229,000
Including Day 1 Reserve Related to Merger [Member]
Financing Receivable, Allowance for Credit Loss	53,300,000		53,300,000
Unfunded Loan Commitment [Member]
Financing Receivable, before Allowance for Credit Loss	1,070,000,000.00		1,070,000,000.00		710,000,000
Financing Receivable, Allowance for Credit Loss	2,250,000		$ 2,250,000		$ 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	58,000,000
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)	$ 342,000