Pension Plans - Defined benefit plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in benefit obligation:
Benefit obligation at beginning of year	$ 15,455,000	$ 13,359,000
Interest cost	436,000	516,000	$ 463,000
Actuarial loss	1,569,000	1,998,000
Benefits paid	(432,000)	(418,000)
Benefit obligation at end of year	17,028,000	15,455,000	13,359,000
Change in plan assets:
Fair value of plan assets at beginning of year	13,134,000	11,440,000
Actual gain on plan assets	2,009,000	2,108,000
Benefits paid	(432,000)	(418,000)
Other	4,000	4,000
Unfunded status	(2,313,000)	(2,321,000)
Fair value of plan assets at end of year	14,715,000	13,134,000	11,440,000
Amounts recognized in consolidated balance sheets
Accrued liabilities	(1,442,000)	(1,305,000)
Liability for pension benefits	(871,000)	(1,016,000)
Total	(2,313,000)	(2,321,000)
Amounts recognized in accumulated other comprehensive loss that have not been recognized as components of net periodic pension benefit (expense)
Net actuarial loss, pre-tax	6,306,000	6,176,000
Components of net periodic pension benefit
Interest cost	436,000	516,000	463,000
Expected return on plan assets	(740,000)	(727,000)	(699,000)
Recognized net actuarial loss	167,000	149,000	151,000
Total net periodic pension benefit	$ (137,000)	$ (62,000)	$ (85,000)
Net Periodic Pension Cost
Weighted-average discount rate (as a percent)	3.40%	4.40%	3.80%
Expected long-term rate of return on plan assets (as a percent)	5.75%	6.50%	6.50%
Benefit Obligation
Weighted-average discount rate (as a percent)	2.80%	3.40%