SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Definite-Lived Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Net Carrying Amount, Core deposits	$ 30,756
Core Deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount, Core deposits	42,651	$ 38,095
Accumulated Amortization, Core deposits	(11,895)	(7,802)
Net Carrying Amount, Core deposits	$ 30,756	$ 30,293