GOODWILL AND INTANGIBLE ASSETS - Summary of Acquired Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 131,565	$ 131,565
Accumulated Amortization	59,591	39,979
Core deposit premiums
Finite-Lived Intangible Assets [Line Items]
Gross Amount	107,958	107,958
Accumulated Amortization	50,829	34,220
Insurance agent relationships
Finite-Lived Intangible Assets [Line Items]
Gross Amount	22,351	22,351
Accumulated Amortization	8,149	5,355
US Premium Finance trade name
Finite-Lived Intangible Assets [Line Items]
Gross Amount	1,094	1,094
Accumulated Amortization	456	300
Non-compete agreement
Finite-Lived Intangible Assets [Line Items]
Gross Amount	162	162
Accumulated Amortization	$ 157	$ 104