GOODWILL AND OTHER INTANGIBLE ASSETS - Gross Carrying Amounts and Accumulated Amortization (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 42,944	$ 42,944
Accumulated Amortization	(15,413)	(10,423)
Net Carrying Amount	27,531	32,521
Goodwill	370,223	370,223
Goodwill	370,223	370,223
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	29,038	29,038
Accumulated Amortization	(11,496)	(7,969)
Net Carrying Amount	$ 17,542	$ 21,069
Core deposit intangibles | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Life	7 years 8 months 12 days	8 years 4 months 24 days
Other identifiable intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 13,906	$ 13,906
Accumulated Amortization	(3,917)	(2,454)
Net Carrying Amount	$ 9,989	$ 11,452
Other identifiable intangibles | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Life	10 years	10 years 8 months 12 days