GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Carrying amount of intangible assets (incl accumulated amortization) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 121,197	$ 120,684
Accumulated Amortization	74,377	72,249
Net Carrying Amount	46,820	48,435
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	101,185	101,185
Accumulated Amortization	61,477	58,970
Net Carrying Amount	39,708	42,215
Customer relationship intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,177	1,177
Accumulated Amortization	1,018	1,009
Net Carrying Amount	159	168
Mortgage servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	11,781	11,268
Accumulated Amortization	5,563	6,079
Net Carrying Amount	6,218	5,189
Commercial servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,054	7,054
Accumulated Amortization	6,319	6,191
Net Carrying Amount	$ 735	$ 863