PREMISES, EQUIPMENT, AND LEASES - Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Operating lease cost	$ 18,639	$ 19,004
Short-term lease cost	222	52
Variable lease cost	5	291
Less: Sublease income	(28)	(101)
Total operating lease cost	18,838	19,246
Operating cash flows from operating leases	20,244	20,230
ROU assets obtained in exchange for new operating lease liabilities	$ 11,737	$ 8,131
Weighted average remaining lease term for operating leases	7 years 7 months 6 days	8 years 1 month 6 days
Weighted-average discount rate for operating leases (percent)	3.10%	3.20%