Segment and Related Information - Narrative (Details)	12 Months Ended
	Dec. 31, 2020 segment	Dec. 31, 2020 subsidiaryBank	Dec. 31, 2020 bankingOffice	Dec. 31, 2020 subsidiaryTrust	Dec. 31, 2020 office	Dec. 31, 2019 bankingOffice
Segment Reporting Information [Line Items]
Number of reportable business segments	3			3
Number of subsidiary banks (banking office)		4	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