RETIREMENT PLANS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure
Employer contributions	$ 249	$ 456
Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP	1,520	1,450	$ 1,400
Cash contributions to ESOP for employees no longer participating in plan	1,300	1,100	1,100
Pension plans with accumulated benefit obligations in excess of plan assets, aggregate accumulated benefit obligation	81,800	81,500
Pension and other postretirement defined benefit plans, liabilities	7,800	7,500
Accumulated other comprehensive loss	(127,087)	(139,974)	(2,426)
First Financial Corporation Common Stock
Defined Benefit Plan Disclosure
Defined benefit plan assets equity securities	$ 16,300	$ 17,200
Percentage of plan assets (percent)	22.00%	24.00%
Pension Plan
Defined Benefit Plan Disclosure
Employer contributions	$ 126	$ 2,050	2,050
Expected contributions to pension plan in 2014	3,900
Employee Stock Ownership Plan
Defined Benefit Plan Disclosure
Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP	604
Employee Stock Ownership Plan | First Financial Corporation Common Stock
Defined Benefit Plan Disclosure
Defined benefit plan assets equity securities	8,300	7,800
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure
Pension expense	517	751	$ 748
Accumulated other comprehensive income (loss), after tax	926	1,200
Accumulated other comprehensive loss	459	546
Postretirement Health Coverage
Defined Benefit Plan Disclosure
Expected contributions to pension plan in 2014	249
Pension expense	$ 277	$ 300