Pension Plans - Defined benefit plan (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Components of net periodic pension benefit
Interest cost	$ 82,000	$ 108,000	$ 248,000	$ 325,000
Expected return on plan assets	(207,000)	(185,000)	(623,000)	(556,000)
Recognized net actuarial loss	42,000	41,000	127,000	122,000
Total net periodic pension benefit	(83,000)	$ (36,000)	(248,000)	$ (109,000)
Minimum required pension contributions for 2021	$ 0		$ 0