Pension Plans - Defined benefit plan (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Change in benefit obligation:
Interest cost	$ 108,000	$ 130,000	$ 325,000	$ 387,000
Components of net periodic pension benefit
Interest cost	108,000	130,000	325,000	387,000
Expected return on plan assets	(185,000)	(183,000)	(556,000)	(546,000)
Recognized net actuarial loss	41,000	40,000	122,000	111,000
Total net periodic pension benefit	(36,000)	$ (13,000)	(109,000)	$ (48,000)
Minimum required pension contributions for 2020	$ 0		$ 0