REGULATORY MATTERS - Regulatory Capital Requirements under Banking Regulations (Details) $ in Thousands	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Corporation [Member]
Capital [Abstract]
Capital	$ 561,347	$ 533,599
Capital to Risk Weighted Assets (percent)	0.1461	0.1563
Capital Required for Capital Adequacy	$ 403,400	$ 358,575
Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	0.10500	0.10500
Common equity capital	$ 521,568	$ 490,842
Common Equity Capital to Risk Weighted Assets	0.1358%	0.1437%
Common equity capital required for capital adequacy	$ 268,933	$ 239,050
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	$ 521,568	$ 490,842
Tier One Risk Based Capital to Risk Weighted Assets (percent)	0.1358	0.1437
Tier One Risk Based Capital Required for Capital Adequacy	$ 326,562	$ 290,275
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	$ 521,568	$ 490,842
Tier One Leverage Capital to Average Assets (percent)	0.1078	0.0983
Tier One Leverage Capital Required for Capital Adequacy	$ 193,476	$ 199,702
Tier One Leverage Capital Required for Capital Adequacy to Average Assets (percent)	0.0400	0.0400
First Financial Bank
Capital [Abstract]
Capital	$ 498,246	$ 487,416
Capital to Risk Weighted Assets (percent)	0.1314	0.1478
Capital Required for Capital Adequacy	$ 398,179	$ 346,248
Capital Required for Capital Adequacy to Risk Weighted Assets (percent)	0.10500	0.10500
Capital Required to be Well Capitalized	$ 379,219	$ 329,760
Capital Required to be Well Capitalized to Risk Weighted Assets (percent)	0.1000	0.1000
Common equity capital	$ 458,467	$ 446,189
Common Equity Capital to Risk Weighted Assets	0.1209%	0.1353%
Common equity capital required for capital adequacy	$ 265,453	$ 230,832
Common Equity Capital required for Capital Adequacy to Risk Weighted Assets	7.00%	7.00%
Common equity capital required to be well capitalized	$ 246,492	$ 214,344
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	$ 458,467	$ 446,189
Tier One Risk Based Capital to Risk Weighted Assets (percent)	0.1209	0.1353
Tier One Risk Based Capital Required for Capital Adequacy with Buffer	$ 322,336	$ 280,296
Tier One Risk Based Capital Required for Capital Adequacy with Buffer to Risk Weighted Assets (percent)	0.08500	0.08500
Tier One Risk Based Capital Required to be Well Capitalized	$ 303,375	$ 263,808
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	$ 458,467	$ 446,189
Tier One Leverage Capital to Average Assets (percent)	0.0950	0.0918
Tier One Leverage Capital Required for Capital Adequacy	$ 193,073	$ 194,405
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	$ 241,341	$ 243,006
Tier One Leverage Capital Required to be Well Capitalized to Average Assets (percent)	0.0500	0.0500