Accumulated Other Comprehensive Income (Loss) - Summary of Amounts Reclassified Out of Each Component of AOCI (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits		$ (3,454)	$ (3,211)	$ (7,104)	$ (6,190)
Provision for income taxes		(175)	(166)	(426)	(348)
Net income attributable to Pathfinder Bancorp Inc.		607	945	1,121	1,949
Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-Sale Securities [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net gains (losses) on sales and redemptions of investment securities	[1]	32	(22)	111	(129)
Provision for income taxes	[1]	(7)	6	(23)	34
Net income attributable to Pathfinder Bancorp Inc.	[1]	25	(16)	88	(95)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Retirement Plans [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	[1],[2]	(84)	(43)	(168)	(86)
Provision for income taxes	[1]	18	11	37	22
Net income attributable to Pathfinder Bancorp Inc.	[1]	$ (66)	$ (32)	$ (131)	$ (64)

[1] Amounts in parentheses indicates debits in net income.

[2] These items are included in net periodic pension cost.