GOODWILL AND INTANGIBLE ASSETS - Summary of Acquired Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 80,955	$ 26,250
Accumulated Amortization	22,266	12,754
Core deposit premiums
Finite-Lived Intangible Assets [Line Items]
Gross Amount	57,348	26,250
Accumulated Amortization	19,512	12,754
Insurance agent relationships
Finite-Lived Intangible Assets [Line Items]
Gross Amount	22,351	0
Accumulated Amortization	2,561	0
US Premium Finance trade name
Finite-Lived Intangible Assets [Line Items]
Gross Amount	1,094	0
Accumulated Amortization	143	0
Non-compete agreement
Finite-Lived Intangible Assets [Line Items]
Gross Amount	162	0
Accumulated Amortization	$ 50	$ 0