RETIREMENT PLANS: (Details Textual) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	100.00%	100.00%
Employer contributions	$ 2,092	$ 2,614
Cash contributions to ESOP	(1,251)	(1,087)	$ (1,062)
Cash contributions to ESOP for employees no longer participating in plan	926	735	676
Amounts that will be amortized from accumulated other comprehensive income (loss) in next fiscal year	2,000
Pension plans with accumulated benefit obligations in excess of plan assets, aggregate accumulated benefit obligation	98,400	83,900
Pension and other postretirement defined benefit plans, liabilities	6,700	5,000
Accumulated other comprehensive income (loss)	(7,501)	(23,454)	(14,704)
Prior service cost (credit)	1
First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 19,800	$ 18,000
Percentage of plan assets (percent)	27.00%	28.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	63.00%	65.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	33.00%	31.00%
Other Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	4.00%	4.00%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1,770	$ 2,260	2,550
Expected contributions to pension plan in 2014	$ 3,700
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	$ (736)
Employee Stock Ownership Plan \| First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 5,100	$ 3,800
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 [Member]
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]
Amounts that will be amortized from accumulated other comprehensive income (loss) in next fiscal year	$ 247
Pension expense	339	$ 411	321
Accumulated other comprehensive income (loss), after tax	2,350	1,070
Accumulated other comprehensive income (loss)	(27)	(615)
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contributions to pension plan in 2014	251
Pension expense	315	325
Amortization of transition obligations (assets)	$ (16)	$ 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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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 included in accumulated other comprehensive (income) loss for prior service cost (credit) expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position.

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

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