RETIREMENT PLANS (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure
Defined benefit plan percentage of plan assets	100.00%	100.00%
Employer contributions	$ 456	$ 2,358
Cash contributions to ESOP	(1,450)	(1,400)	$ (1,470)
Cash contributions to ESOP for employees no longer participating in plan	1,100	1,100	1,200
Pension plans with accumulated benefit obligations in excess of plan assets, aggregate accumulated benefit obligation	81,500	102,400
Pension and other postretirement defined benefit plans, liabilities	7,500	8,800
Accumulated other comprehensive income/(loss)	(139,974)	(2,426)	9,764
First Financial Corporation Common Stock
Defined Benefit Plan Disclosure
Defined benefit plan assets equity securities	$ 17,200	$ 18,100
Percentage of plan assets (percent)	24.00%	21.00%
Equity Securities
Defined Benefit Plan Disclosure
Defined benefit plan percentage of plan assets	63.00%	63.00%
Debt securities
Defined Benefit Plan Disclosure
Defined benefit plan percentage of plan assets	34.00%	32.00%
Other securities
Defined Benefit Plan Disclosure
Defined benefit plan percentage of plan assets	3.00%	5.00%
Pension Plan
Defined Benefit Plan Disclosure
Employer contributions	$ 126	$ 2,050	4,440
Expected contributions to pension plan in 2014	$ 0
Employee Stock Ownership Plan
Defined Benefit Plan Disclosure
Defined benefit plan percentage of plan assets	100.00%	100.00%
Cash contributions to ESOP	$ (642)
Employee Stock Ownership Plan \| First Financial Corporation Common Stock
Defined Benefit Plan Disclosure
Defined benefit plan assets equity securities	$ 7,800	$ 7,200
Employee Stock Ownership Plan \| Equity Securities
Defined Benefit Plan Disclosure
Defined benefit plan percentage of plan assets	99.00%	98.00%
Employee Stock Ownership Plan \| Debt securities
Defined Benefit Plan Disclosure
Defined benefit plan percentage of plan assets	0.00%	0.00%
Employee Stock Ownership Plan \| Other securities
Defined Benefit Plan Disclosure
Defined benefit plan percentage of plan assets	1.00%	2.00%
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure
Pension expense	$ 751	$ 748	539
Accumulated other comprehensive income (loss), after tax	1,200	3,200
Accumulated other comprehensive income/(loss)	(546)	(212)
Postretirement Health Coverage
Defined Benefit Plan Disclosure
Expected contributions to pension plan in 2014	245
Pension expense	300	259
Amortization of transition obligations (assets)	$ 0	$ 0	$ 0

Defined Benefit Plan, Expected Future Contributions, Next Twelve Months

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Assets, equity stocks, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

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The percentage of plan assets during the period.

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Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP for Employees no Longer Participating in Plan

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The percentage of total plan assets during the period.

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Amount, after tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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Amount of transition asset (obligation) recognized in net periodic benefit (cost) credit of defined benefit plan.

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

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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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Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

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This item represents the amount of cash contributions during the period made by the entity to the Employee Stock Ownership Plan (ESOP).

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans.

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Amount of cost (reversal of cost) for pension benefits. Excludes other postretirement benefits.

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