REGULATORY MATTERS: (Details) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Corporation [Member]
Capital [Abstract]
Capital	$ 538,440	$ 495,817
Capital to Risk Weighted Assets (percent)	0.1740	0.1616
Capital Required for Capital Adequacy	$ 324,849	$ 322,091
Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	0.10500	0.10500
Common equity capital	$ 499,664	$ 475,874
Commen Equity Capital to Risk Weighted Assets	16.15%	15.51%
Common equity capital required for capital adequacy	$ 216,566	$ 214,728
Common Equity Capital required for Capital Adequacy to Risk Weighted Assets	7.00%	7.00%
Tier One Risk Based Capital [Abstract]
Tier One Risk Based Capital	$ 499,664	$ 475,874
Tier One Risk Based Capital to Risk Weighted Assets (percent)	0.1615	0.1551
Tier One Risk Based Capital Required for Capital Adequacy	$ 262,973	$ 260,741
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	0.08500	0.08500
Tier One Leverage Capital [Abstract]
Tier One Leverage Capital	$ 499,664	$ 475,874
Tier One Leverage Capital to Average Assets (percent)	0.1124	0.1204
Tier One Leverage Capital Required for Capital Adequacy	$ 177,781	$ 158,081
Tier One Leverage Capital Required for Capital Adequacy to Average Assets (percent)	0.0400	0.0400
First Financial Bank [Member]
Capital [Abstract]
Capital	$ 507,869	$ 472,995
Capital to Risk Weighted Assets (percent)	0.1703	0.1591
Capital Required for Capital Adequacy	$ 313,075	$ 312,111
Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	0.10500	0.10500
Capital Required to be Well Capitalized	$ 298,166	$ 297,249
Capital Required to be Well Capitalized to Risk Weighted Assets (percent)	0.1000	0.1000
Common equity capital	$ 470,551	$ 457,649
Commen Equity Capital to Risk Weighted Assets	15.78%	15.40%
Common equity capital required for capital adequacy	$ 208,716	$ 208,074
Common Equity Capital required for Capital Adequacy to Risk Weighted Assets	7.00%	7.00%
Common equity capital required to be well capitalized	$ 193,808	$ 193,212
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	$ 470,551	$ 457,649
Tier One Risk Based Capital to Risk Weighted Assets (percent)	0.1578	0.1540
Tier One Risk Based Capital Required for Capital Adequacy	$ 253,441	$ 252,661
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	0.08500	0.08500
Tier One Risk Based Capital Required to be Well Capitalized	$ 238,533	$ 237,799
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets (percent)	0.0800	0.0800
Tier One Leverage Capital [Abstract]
Tier One Leverage Capital	$ 470,551	$ 457,649
Tier One Leverage Capital to Average Assets (percent)	0.1090	0.1193
Tier One Leverage Capital Required for Capital Adequacy	$ 172,728	$ 153,453
Tier One Leverage Capital Required for Capital Adequacy to Average Assets (percent)	0.0400	0.0400
Tier One Leverage Capital Required to be Well Capitalized	$ 215,910	$ 191,816
Tier One Leverage Capital Required to be Well Capitalized to Average Assets (percent)	0.0500	0.0500