Components of Net Periodic Benefit Cost - Employer Contributions (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Defined Benefit Plan Disclosure
Amount of potential discretionary contributions accrued to a defined contribution plan	$ 2,400	$ 2,400
Pension plan
Defined Benefit Plan Disclosure
Expected future benefit payment			$ 570
Contributions by employer	905
Pension plan \| Employee Stock Option
Defined Benefit Plan Disclosure
Expected future benefit payment			563
Contributions by employer	0
Postretirement Health Coverage
Defined Benefit Plan Disclosure
Expected future benefit payment			$ 243
Contributions by employer	$ 188

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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+ Details

- Definition

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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+ Details

FIRST FINANCIAL CORP /IN/ 10-Q