Represents the benchmark for the maximum number of shares awarded required for maximum vesting rights.

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Represents the benchmark for the minimum number of shares awarded required for minimum vesting rights.

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No definition available.

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- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

Number of shares authorized for issuance under share-based payment arrangement.

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- Definition

Gross number of share options (or share units) granted during the period.

+ References

No definition available.

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- Definition

Percentage of vesting of award under share-based payment arrangement.

+ References

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Number of options vested.

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No definition available.

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- Definition

Purchase price of common stock expressed as a percentage of its fair value.

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No definition available.

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- Definition

Number of shares issued in lieu of cash for services contributed to the entity. Number of shares includes, but is not limited to, shares issued for services contributed by vendors and founders.

+ References

No definition available.

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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
	Aug. 27, 2020	Jun. 01, 2020	May 23, 2019	Oct. 25, 2018	May 24, 2018	May 25, 2017	Jun. 01, 2014	May 31, 2019	May 31, 2021	May 31, 2021	Dec. 31, 2020	May 31, 2020	Dec. 31, 2019	May 31, 2019	Dec. 31, 2018	May 31, 2018	Dec. 31, 2015	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares)											361,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)											0		0		0
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)		16,000	12,000		11,000	12,000
Share-based Payment Arrangement, Expense												$ 0.4		$ 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)	2,000			1,000
Share-based Payment Arrangement, Expense								$ 0.1
Bank Board Members [Member] \| Forecast [Member]
Share-based Payment Arrangement, Expense									$ 0.1
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)											121,518		84,596		91,400
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares)											35,997		3,531		12,278
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)											31,295		23,596		24,633
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)											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)											14,681