Segment and Related Information - Narrative (Details) - 12 months ended Dec. 31, 2017	segment	banking_office	subsidiary_bank	subsidiary_trust	office
Segment Reporting Information [Line Items]
Number of reportable business segments | segment	3
Number of subsidiary banks		4	3	4
Ithaca, NY
Segment Reporting Information [Line Items]
Number of banking offices		14
Genesee Valley Region, New York State
Segment Reporting Information [Line Items]
Number of banking offices		17
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
Tompkins County, New York
Segment Reporting Information [Line Items]
Nature of operations, number of offices | office					1