Supplemental Compensation Plans, Components of Net Periodic Benefit Cost and Additional Information (Details) - Pension Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Salary Continuation Plan [Member]
Components of net periodic benefit cost [Abstract]
Service cost	$ 256	$ 213	$ 174
Interest cost	180	221	185
Amortization of prior service cost	2	2	2
Recognized actuarial loss (gain)	115	87	101
Net periodic benefit cost	553	523	462
Additional Information [Abstract]
Minimum benefit obligation at year end	7,127	5,871	5,322	$ 5,419
Increase (decrease) in minimum liability included in other comprehensive income (loss)	974	298	(287)
Directors' Retirement Plan [Member]
Components of net periodic benefit cost [Abstract]
Service cost	0	2	12
Interest cost	19	28	25
Recognized actuarial loss (gain)	0	0	0
Net periodic benefit cost	19	30	37
Additional Information [Abstract]
Minimum benefit obligation at year end	831	820	787	$ 856
Increase (decrease) in minimum liability included in other comprehensive income (loss)	$ 52	$ 62	$ (36)