GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Carrying amount of intangible assets (incl accumulated amortization) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 121,387	$ 120,684
Accumulated Amortization	77,734	72,249
Net Carrying Amount	43,653	48,435
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	101,185	101,185
Accumulated Amortization	63,883	58,970
Net Carrying Amount	37,302	42,215
Customer relationship intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,177	1,177
Accumulated Amortization	1,027	1,009
Net Carrying Amount	150	168
Mortgage servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	11,971	11,268
Accumulated Amortization	6,412	6,079
Net Carrying Amount	5,559	5,189
Commercial servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,054	7,054
Accumulated Amortization	6,412	6,191
Net Carrying Amount	$ 642	$ 863