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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Premises and Equipment (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Other Assets    
Property, Plant and Equipment [Line Items]    
Premises held-for-sale $ 1.1 $ 5.2
Minimum | Buildings and improvements, including leasehold improvements    
Property, Plant and Equipment [Line Items]    
Useful life 10 years  
Minimum | Furniture and equipment    
Property, Plant and Equipment [Line Items]    
Useful life 3 years  
Maximum | Buildings and improvements, including leasehold improvements    
Property, Plant and Equipment [Line Items]    
Useful life 40 years  
Maximum | Furniture and equipment    
Property, Plant and Equipment [Line Items]    
Useful life 15 years