GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Carrying amount of intangible assets (incl accumulated amortization) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 113,250	$ 112,836
Accumulated Amortization	62,323	57,412
Net Carrying Amount	50,927	55,424
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	96,821	96,821
Accumulated Amortization	51,309	48,338
Net Carrying Amount	45,512	48,483
Customer relationship intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,177	1,177
Accumulated Amortization	982	972
Net Carrying Amount	195	205
Mortgage servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	8,262	7,886
Accumulated Amortization	4,137	2,265
Net Carrying Amount	4,125	5,621
Commercial servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6,990	6,952
Accumulated Amortization	5,895	5,837
Net Carrying Amount	$ 1,095	$ 1,115