Segment and Related Information - Narrative (Details)	9 Months Ended
	Sep. 30, 2018 subsidiary office banking_office segment
Segment Reporting Information [Line Items]
Number of reportable segments | segment	3
Nature of operations, number of unconsolidated subsidiary trusts | subsidiary	4
Ithaca, NY
Segment Reporting Information [Line Items]
Nature of operations, number of banking offices	14
Genesee Valley Region, New York State
Segment Reporting Information [Line Items]
Nature of operations, number of banking offices	17
Counties North of New York City
Segment Reporting Information [Line Items]
Nature of operations, number of banking offices	14
Southeastern Pennsylvania
Segment Reporting Information [Line Items]
Nature of operations, number of banking offices	21
Western New York
Segment Reporting Information [Line Items]
Number of offices | office	5
Tompkins County, New York
Segment Reporting Information [Line Items]
Number of offices | office	1
Montgomery County, Pennsylvania
Segment Reporting Information [Line Items]
Number of offices | office	1
Tompkins Insurance Agencies Inc [Member]
Segment Reporting Information [Line Items]
Noncontrolling interest, ownership percentage by parent	100.00%