RETIREMENT PLANS: (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	100.00%	100.00%
Employer contributions	$ 3,184	$ 2,389
Cash contributions to ESOP	(1,361)	(1,294)	$ (1,253)
Cash contributions to ESOP for employees no longer participating in plan	872	746	716
Amounts that will be amortized from accumulated other comprehensive income (loss) in next fiscal year	1,200
Amortization of prior service cost (credit)	1
Pension plans with accumulated benefit obligations in excess of plan assets, aggregate accumulated benefit obligation	88,500	85,100
Pension and other postretirement defined benefit plans, liabilities	3,600	3,700
Accumulated other comprehensive income (loss)	(14,164)	(9,401)	(14,529)
First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 26,700	$ 20,400
Percentage of plan assets (percent)	38.00%	34.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	68.00%	63.00%
Target plan asset allocations range minimum (percent)	40.00%
Target plan asset allocations range maximum (percent)	65.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	29.00%	35.00%
Target plan asset allocations range minimum (percent)	35.00%
Target plan asset allocations range maximum (percent)	60.00%
Other Security Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations range minimum (percent)	0.00%
Target plan asset allocations range maximum (percent)	10.00%
Other Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	3.00%	2.00%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 2,700	$ 1,840	3,240
Expected contributions to pension plan in 2014	$ 2,500
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	$ (858)
Employee Stock Ownership Plan | First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 4,100	$ 2,100
Employee Stock Ownership Plan | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	99.00%	100.00%
Target plan asset allocations range minimum (percent)	95.00%
Target plan asset allocations range maximum (percent)	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%
Target plan asset allocations range minimum (percent)	0.00%
Target plan asset allocations range maximum (percent)	0.00%
Employee Stock Ownership Plan | Other Security Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations range minimum (percent)	0.00%
Target plan asset allocations range maximum (percent)	5.00%
Employee Stock Ownership Plan | Other Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	1.00%	0.00%
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	$ 3
Pension expense	418	$ 437	268
Accumulated other comprehensive income (loss), after tax	332	900
Accumulated other comprehensive income (loss)	(174)	(318)
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contributions to pension plan in 2014	260
Pension expense	273	269
Amortization of transition obligations (assets)	$ 0	$ 0	$ 0