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Note 11 - Stock-based Compensation (Details Textual) - USD ($) $ in Millions								8 Months Ended	9 Months Ended	12 Months Ended								36 Months Ended
	May 27, 2021	Aug. 27, 2020	Jun. 01, 2020	May 23, 2019	Oct. 25, 2018	May 24, 2018	Jun. 01, 2014	May 31, 2019	May 31, 2021	May 31, 2022	Dec. 31, 2021	May 31, 2021	Dec. 31, 2020	May 31, 2020	Dec. 31, 2019	May 31, 2019	Dec. 31, 2015	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares)											187,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)											0		0		0
Stock Issued During Period, Shares, Issued for Services (in shares)											10,489		17,716		11,905
Mercantile Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)											0		0		0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value											$ 0.1
Firstbank Corporation [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)							282,200
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares (in shares)							258,400										23,800
First Bank Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)											10 years
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Number of Shares for Maximum Vesting Rights (in shares)											50
Share-based Compensation Arrangement by Share-based Payment Award, Minimum Number of Shares for Minimum Vesting Rights (in shares)											50
First Bank Employees [Member] | Grant Per Annum Starting One Year From Date of Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage											20.00%
First Bank Employees [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)											1 year
First Bank Employees [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)											5 years
Director [Member]
Stock Issued During Period, Shares, Issued for Services (in shares)	10,000		16,000	12,000		11,000
Share-based Payment Arrangement, Expense												$ 0.3		$ 0.4		$ 0.4
Director [Member] | Forecast [Member]
Share-based Payment Arrangement, Expense										$ 0.3
Bank Board Members [Member]
Stock Issued During Period, Shares, Issued for Services (in shares)					1,000
Share-based Payment Arrangement, Expense								$ 0.1	$ 0.1
Corporate and Bank Board Members [Member]
Stock Issued During Period, Shares, Issued for Services (in shares)		2,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)											3 years
Restricted Stock [Member] | Mercantile Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)											163,000		121,518		84,596
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares)											11,873		35,997		3,531
Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)																		2 years
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent																		110.00%
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)																		7 years
Performance Shares [Member] | Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)											22,703		31,295		23,596
Performance Shares [Member] | Executive Officer [Member] | The 2020 Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares)											11,454
Performance Shares [Member] | Executive Officer [Member] | The 2019 Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares)											23,053		13,709
Performance Shares [Member] | Executive Officer [Member] | The 2018 Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares)											22,836		14,681

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