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Segment and Related Information - Narrative (Details) - 12 months ended Dec. 31, 2019
segment
banking_office
subsidiary_bank
office
Segment Reporting Information [Line Items]        
Number of reportable business segments | segment 3      
Number of subsidiary banks (banking office)   4 4  
Ithaca, NY        
Segment Reporting Information [Line Items]        
Number of banking offices   14    
Genesee Valley Region        
Segment Reporting Information [Line Items]        
Number of banking offices   16    
Counties North of New York City        
Segment Reporting Information [Line Items]        
Number of banking offices   14    
Southeastern Pennsylvania        
Segment Reporting Information [Line Items]        
Number of banking offices   20    
Western New York        
Segment Reporting Information [Line Items]        
Nature of operations, number of offices | office       5