Income Taxes (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) limited_partnership	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) limited_partnership	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) limited_partnership	Dec. 31, 2018 USD ($) limited_partnership	Dec. 31, 2017 USD ($)
Federal income tax
Current									$ 3,978	$ 4,407	$ 4,184
Deferred									336	(492)	2,083
Federal income tax expense									4,314	3,915	6,267
State franchise tax									421	391	345
Income tax expense	$ 1,261	$ 1,180	$ 1,180	$ 1,114	$ 1,222	$ 1,088	$ 1,040	$ 956	4,735	4,306	6,612
Reconciliation between US expected tax rate and actual tax expense
Expected tax expense									6,355	5,847	9,170
Non-taxable income									(1,749)	(1,699)	(2,625)
State franchise tax, net of federal tax benefit									332	309	224
Equity compensation									(45)	(55)	(83)
Tax credits, net of amortization									(85)	(85)	(88)
Change in federal tax rate									0	0	134
Other									(73)	(11)	(120)
Income tax expense	1,261	$ 1,180	$ 1,180	$ 1,114	1,222	$ 1,088	$ 1,040	$ 956	4,735	4,306	$ 6,612
Components of deferred tax assets and liabilities
Allowance for loan losses	2,444				2,359				2,444	2,359
Accrued pension and post-retirement	926				955				926	955
Unrealized loss on securities available for sale	0				1,343				0	1,343
Unrealized loss on securities transferred from available for sale to held to maturity	48				52				48	52
Restricted stock grants	203				170				203	170
Core deposit intangible	21				18				21	18
Investment in flow through entities	55				31				55	31
Other assets	0				24				0	24
Total deferred tax asset	3,697				4,952				3,697	4,952
Net deferred loan costs	(1,669)				(1,504)				(1,669)	(1,504)
Depreciation	(1,434)				(1,300)				(1,434)	(1,300)
Unrealized gain on securities available for sale	(972)				0				(972)	0
Goodwill	(120)				(80)				(120)	(80)
Mortgage servicing rights	(325)				(284)				(325)	(284)
Unrealized gain on derivative instruments	(26)				(382)				(26)	(382)
Prepaid expense	(205)				(159)				(205)	(159)
Total deferred tax liability	(4,751)				(3,709)				(4,751)	(3,709)
Net deferred tax asset (liability)	$ 1,054								$ 1,054
Net deferred tax asset (liability)					$ 1,243					$ 1,243
Limited partnerships
Limited partnerships held as investments | limited_partnership	4				3				4	3
Tax credits from investments in limited partnerships									$ 330	$ 210
Amortization of investments in limited partnerships									307	186
Carrying value of investments in limited partnerships	$ 1,951				$ 1,408				1,951	1,408
Total exposure to limited partnerships	$ 1,951				$ 1,408				$ 1,951	$ 1,408