GOODWILL AND OTHER INTANGIBLE ASSETS - Gross Carrying Amounts and Accumulated Amortization (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
	Apr. 01, 2020	Sep. 30, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		$ 42,944	$ 12,156
Accumulated Amortization		(8,769)	(4,315)
Net Carrying Amount		34,175	7,841
Goodwill		370,549	347,149
Goodwill		370,549	347,149
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		29,038	10,678
Accumulated Amortization		(6,841)	(3,689)
Net Carrying Amount		$ 22,197	$ 6,989
Weighted Average Remaining Life		8 years 8 months 12 days	8 years
Core deposit intangibles | Revere acquisition
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Life	9 years 8 months 1 day
Intangible asset acquired	$ 18,400
Other identifiable intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		$ 13,906	$ 1,478
Accumulated Amortization		(1,928)	(626)
Net Carrying Amount		$ 11,978	$ 852
Weighted Average Remaining Life		10 years 10 months 24 days	9 years 8 months 12 days