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Note 1 - Summary of Significant Accounting Policies (Details Textual)	3 Months Ended			9 Months Ended
	Sep. 30, 2021 USD ($)	Mar. 31, 2021	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2020 USD ($)	Apr. 30, 2021 shares	Dec. 31, 2020 USD ($)	Jul. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Oct. 01, 2019 USD ($)	Dec. 31, 2006 USD ($)
Participating Interest In Loans, Losses or Charge-offs				$ 0	$ 0
Stock Issued During Period, Value, New Issues	$ 139,000		$ 143,000	$ 420,000	$ 304,000
Stock Repurchased During Period, Shares (in shares) | shares				223,000
Stock Repurchased During Period, Value	2,664,000			$ 5,561,000
Stock Repurchased, Price Per Share (in dollars per share) | $ / shares				$ 24.94
Stock Repurchase Program, Number of Shares Authorized to be Repurchased (in shares) | shares						390,114
Stock Repurchase Program, Percent of Stock Outstanding						5.00%
Goodwill, Ending Balance	$ 59,946,000			$ 59,946,000			$ 60,506,000
Valley Ridge Financial Corp. [Member]
Goodwill, Ending Balance											$ 13,700,000
County Bank Corp ("County") [Member]
Goodwill, Ending Balance									$ 38,900,000	$ 38,866,000
Community Shores Bank Corp ("Community Shores") [Member]
Goodwill, Ending Balance							$ 7,300,000	$ 7,353,000
Directors' Stock Purchase Plan [Member]
Stock Issued During Period, Shares, New Issues (in shares) | shares				13,092
Stock Issued During Period, Value, New Issues				$ 348,000
Employee Stock Purchase Plan [Member]
Stock Issued During Period, Shares, New Issues (in shares) | shares				4,718
Stock Issued During Period, Value, New Issues				$ 104,000
Participating Interests in Mortgage Loans [Member] | Credit Concentration Risk [Member]
Number Of Mortgage Bankers		11
Number of Participating Interest In Loans To Other Financial Institutions Over 30 Days	0			0			26
Number of Participating Interest In Loans To Other Financial Institutions							218
Participating Interests in Mortgage Loans [Member] | Credit Concentration Risk [Member] | Past Due 30 Days or More [Member]
Participating Mortgage Loans, Mortgage Obligations, Amount							$ 7,900,000
Participating Interests in Mortgage Loans [Member] | Credit Concentration Risk [Member] | Eleven Mortgage Brokers [Member]
Concentration Risk, Percentage		17.00%

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