Restricted Stock Plan (Summary of Nonvested Awards) (Details) - Restricted Stock - Isabella Bank Corporation Restricted Stock Plan - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	17,518	30,777	27,072	27,072
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Nonvested	$ 377	$ 683	$ 592	$ 592
Share-Based Compensation Arrangement by Share-Based Payment Award, Non-Option Equity Instruments, Period Increase (Decrease)	6,686	3,705
Shares Granted, Value, Share-based Payment Arrangement, before Forfeiture	$ 130	$ 91
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	(16,240)	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested	$ (345)	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Intrinsic Value, Amount Per Share	$ 0	$ 0

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 non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Increase (decrease) in the number of shares under non-option equity instrument agreements.

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Intrinsic value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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No definition available.

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Intrinsic value of vested award under share-based payment arrangement. Excludes share and unit options.

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Value of forfeited shares granted under share-based payment arrangement. Excludes employee stock ownership plan (ESOP).

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Value, before forfeiture, of shares granted under share-based payment arrangement. Excludes employee stock ownership plan (ESOP).

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