RETIREMENT PLANS: (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	100.00%	100.00%
Employer contributions	$ 2,358	$ 4,749
Cash contributions to ESOP	(1,402)	(1,471)	$ (1,251)
Cash contributions to ESOP for employees no longer participating in plan	1,100	1,200	926
Pension plans with accumulated benefit obligations in excess of plan assets, aggregate accumulated benefit obligation	102,400	105,200
Pension and other postretirement defined benefit plans, liabilities	8,800	8,400
Accumulated other comprehensive income (loss)	(2,426)	9,764	(7,501)
First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 18,100	$ 16,500
Percentage of plan assets (percent)	21.00%	20.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	63.00%	63.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	32.00%	31.00%
Other securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	5.00%	6.00%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 2,050	$ 4,440	1,770
Expected contributions to pension plan in 2014	$ 250
Employee Stock Ownership Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	100.00%	100.00%
Cash contributions to ESOP	$ (703)
Employee Stock Ownership Plan | First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 7,200	$ 5,500
Employee Stock Ownership Plan | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	98.00%	99.00%
Employee Stock Ownership Plan | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	0.00%	0.00%
Employee Stock Ownership Plan | Other securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	2.00%	1.00%
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Pension expense	$ 748	$ 539	339
Accumulated other comprehensive income (loss), after tax	3,200	3,600
Accumulated other comprehensive income (loss)	(212)	(266)
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contributions to pension plan in 2014	248
Pension expense	259	305
Amortization of transition obligations (assets)	$ 0	$ 0	$ (16)