GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Carrying amount of intangible assets (incl accumulated amortization) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 121,739	$ 120,684
Accumulated Amortization	80,231	72,249
Net Carrying Amount	41,508	48,435
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	101,185	101,185
Accumulated Amortization	66,169	58,970
Net Carrying Amount	35,016	42,215
Customer relationship intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,177	1,177
Accumulated Amortization	1,036	1,009
Net Carrying Amount	141	168
Mortgage servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	12,323	11,268
Accumulated Amortization	6,522	6,079
Net Carrying Amount	5,801	5,189
Commercial servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,054	7,054
Accumulated Amortization	6,504	6,191
Net Carrying Amount	$ 550	$ 863