RETIREMENT PLANS: (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	100.00%	100.00%
Employer contributions	$ 2,933	$ 3,184
Cash contributions to ESOP	(1,062)	(1,361)	$ (1,294)
Cash contributions to ESOP for employees no longer participating in plan	676	872	746
Amounts that will be amortized from accumulated other comprehensive income (loss) in next fiscal year				$ 1,700
Pension plans with accumulated benefit obligations in excess of plan assets, aggregate accumulated benefit obligation	93,300	88,500
Pension and other postretirement defined benefit plans, liabilities	4,400	3,600
Accumulated other comprehensive income (loss)	(14,704)	(14,164)	(9,401)
Prior service cost (credit)				1
First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 21,400	$ 26,700
Percentage of plan assets (percent)	30.00%	38.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	72.00%	68.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	25.00%	29.00%
Other Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	3.00%	3.00%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 2,550	$ 2,700	1,840
Expected contributions to pension plan in 2014	$ 2,300
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	$ (813)
Employee Stock Ownership Plan | First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 3,800	$ 4,100
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				$ 51
Pension expense	$ 321	$ 418	$ 437
Accumulated other comprehensive income (loss), after tax	859	332
Accumulated other comprehensive income (loss)	(257)	(174)
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contributions to pension plan in 2014	264
Pension expense	$ 298	$ 273