GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Carrying amount of intangible assets (incl accumulated amortization) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 115,469	$ 112,836
Accumulated Amortization	69,197	57,412
Net Carrying Amount	46,272	55,424
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	96,821	96,821
Accumulated Amortization	56,478	48,338
Net Carrying Amount	40,343	48,483
Customer relationship intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,177	1,177
Accumulated Amortization	1,000	972
Net Carrying Amount	177	205
Mortgage servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	10,458	7,886
Accumulated Amortization	5,635	2,265
Net Carrying Amount	4,823	5,621
Commercial servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,013	6,952
Accumulated Amortization	6,084	5,837
Net Carrying Amount	$ 929	$ 1,115