GOODWILL AND INTANGIBLE ASSETS - Summary of Other Intangible Assets (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Intangible Assets	$ 118,425	$ 118,425	$ 33,024
Accumulated Amortization	(19,533)	(19,533)	(13,008)
Total	98,892	98,892	20,016
Core deposits
Finite-Lived Intangible Assets [Line Items]
Gross Intangible Assets	95,711	95,711	10,658
Accumulated Amortization	(10,969)	(10,969)	(5,285)
Total	84,742	$ 84,742	$ 5,373
Amortization Period		10 years	10 years
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Intangible Assets	17,561	$ 17,561	$ 17,561
Accumulated Amortization	(7,015)	(7,015)	(5,815)
Total	10,546	$ 10,546	$ 11,746
Customer relationships | Minimum
Finite-Lived Intangible Assets [Line Items]
Amortization Period		7 years	7 years
Customer relationships | Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization Period		15 years	15 years
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Intangible Assets	221	$ 221	$ 221
Accumulated Amortization	(135)	(135)	(101)
Total	86	$ 86	$ 120
Amortization Period		5 years	5 years
Loan servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Intangible Assets	4,932	$ 4,932	$ 2,652
Accumulated Amortization	(1,414)	(1,414)	(1,301)
Total	3,518	3,518	$ 1,351
Impairment loss	$ 200	$ 500
Loan servicing rights | Minimum
Finite-Lived Intangible Assets [Line Items]
Amortization Period		10 years	10 years
Loan servicing rights | Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization Period		25 years	30 years
Favorable lease asset
Finite-Lived Intangible Assets [Line Items]
Gross Intangible Assets			$ 1,932
Accumulated Amortization			(506)
Total			$ 1,426
Favorable lease asset | Minimum
Finite-Lived Intangible Assets [Line Items]
Amortization Period			10 months
Favorable lease asset | Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization Period			18 years