RETIREMENT PLANS: (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	100.00%	100.00%
Employer contributions	$ 2,389	$ 3,779
Cash contributions to ESOP	(1,294)	(1,253)	$ (1,218)
Cash contributions to ESOP for employees no longer participating in plan	746	716	629
Amounts that will be amortized from accumulated other comprehensive income (loss) in next fiscal year	1,900
Amortization of prior service cost (credit)	1
Pension plans with accumulated benefit obligations in excess of plan assets, aggregate accumulated benefit obligation	85,100	91,500
Pension and other postretirement defined benefit plans, liabilities	3,700	3,600
Accumulated other comprehensive income (loss)	(9,401)	(14,529)	(13,969)
Benefit Obligation Estimate	8,500
First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 20,400	$ 22,500
Percentage of plan assets (percent)	34.00%	36.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	63.00%	59.00%
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	35.00%	38.00%
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	2.00%	3.00%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1,840	$ 3,240	2,110
Expected contributions to pension plan in 2014	$ 2,700
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	$ (1,100)
Employee Stock Ownership Plan | First Financial Corporation Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assets equity securities	$ 2,100	$ 1,400
Employee Stock Ownership Plan | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Percentage Of Plan Assets	100.00%	99.00%
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	0.00%	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	$ 57
Pension expense	437	$ 268
Accumulated other comprehensive income (loss), after tax	900	1,200
Accumulated other comprehensive income (loss)	(318)	(521)
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contributions to pension plan in 2014	262
Pension expense	269	252
Amortization of transition obligations (assets)	$ 0	$ 0	$ 60