REGULATORY MATTERS: (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Corporation [Member]
Capital [Abstract]
Capital	$ 412,525	$ 411,713
Capital to Risk Weighted Assets (percent)	17.88%	18.26%
Capital Required for Capital Adequacy	$ 213,446	$ 194,435
Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	9.25%	8.625%
Common equity capital	$ 392,615	$ 392,939
Commen Equity Capital to Risk Weighted Assets	17.01%	17.43%
Common equity capital required for capital adequacy	$ 132,682	$ 115,534
Common Equity Capital required for Capital Adequacy to Risk Weighted Assets	5.75%	5.125%
Tier One Risk Based Capital [Abstract]
Tier One Risk Based Capital	$ 392,615	$ 392,939
Tier One Risk Based Capital to Risk Weighted Assets (percent)	17.01%	17.43%
Tier One Risk Based Capital Required for Capital Adequacy	$ 167,295	$ 149,348
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	7.25%	6.625%
Tier One Leverage Capital [Abstract]
Tier One Leverage Capital	$ 392,615	$ 392,939
Tier One Leverage Capital to Average Assets (percent)	13.31%	13.39%
Tier One Leverage Capital Required for Capital Adequacy	$ 117,956	$ 117,376
Tier One Leverage Capital Required for Capital Adequacy to Average Assets (percent)	4.00%	4.00%
First Financial Bank [Member]
Capital [Abstract]
Capital	$ 386,593	$ 384,522
Capital to Risk Weighted Assets (percent)	17.30%	17.64%
Capital Required for Capital Adequacy	$ 206,745	$ 188,003
Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	9.25%	8.625%
Capital Required to be Well Capitalized	$ 223,508	$ 217,974
Capital Required to be Well Capitalized to Risk Weighted Assets (percent)	10.00%	10.00%
Common equity capital	$ 370,061	$ 368,797
Commen Equity Capital to Risk Weighted Assets	16.56%	16.92%
Common equity capital required for capital adequacy	$ 128,517	$ 111,712
Common Equity Capital required for Capital Adequacy to Risk Weighted Assets	5.75%	5.125%
Common equity capital required to be well capitalized	$ 145,280	$ 141,683
Common Equity Capital required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier One Risk Based Capital [Abstract]
Tier One Risk Based Capital	$ 370,061	$ 368,797
Tier One Risk Based Capital to Risk Weighted Assets (percent)	16.56%	16.92%
Tier One Risk Based Capital Required for Capital Adequacy	$ 162,044	$ 144,408
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	7.25%	6.625%
Tier One Risk Based Capital Required to be Well Capitalized	$ 178,807	$ 174,379
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets (percent)	8.00%	8.00%
Tier One Leverage Capital [Abstract]
Tier One Leverage Capital	$ 370,061	$ 368,797
Tier One Leverage Capital to Average Assets (percent)	12.81%	12.82%
Tier One Leverage Capital Required for Capital Adequacy	$ 115,553	$ 115,047
Tier One Leverage Capital Required for Capital Adequacy to Average Assets (percent)	4.00%	4.00%
Tier One Leverage Capital Required to be Well Capitalized	$ 144,441	$ 143,809
Tier One Leverage Capital Required to be Well Capitalized to Average Assets (percent)	5.00%	5.00%