GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Carrying amount of intangible assets (incl accumulated amortization) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 133,192	$ 133,879
Accumulated Amortization	59,587	55,328
Net Carrying Amount	73,605	78,551
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	83,640	83,640
Accumulated Amortization	39,236	36,403
Net Carrying Amount	44,404	47,237
Customer relationship intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,177	1,177
Accumulated Amortization	944	935
Net Carrying Amount	233	242
Mortgage servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	41,473	42,228
Accumulated Amortization	14,001	12,865
Net Carrying Amount	27,472	29,363
Commercial servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6,902	6,834
Accumulated Amortization	5,406	5,125
Net Carrying Amount	$ 1,496	$ 1,709