Pension Plans - Funded status and amounts recognized (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Benefit obligation at beginning of year	$ 23,187,000	$ 22,358,000
Interest cost	873,000	867,000	$ 948,000
Actuarial loss	1,597,000	502,000
Benefits paid	(629,000)	(540,000)
Benefit obligation at end of year	25,028,000	23,187,000	22,358,000
Change in plan assets:
Fair value of plan assets at beginning of year	14,960,000	14,442,000
Actual gain on plan assets	2,436,000	848,000
Employer contribution	290,000	210,000
Benefits paid	(629,000)	(540,000)
Fair value of plan assets at end of year	17,057,000	14,960,000	14,442,000
Unfunded status	(7,971,000)	(8,227,000)
Obligations recognized in consolidated balance sheets
Accrued liabilities	(488,000)	(452,000)
Liability for pension benefits	(7,483,000)	(7,775,000)
Pension liabilities recognized in consolidated balance sheet	(7,971,000)	(8,227,000)
Amounts recognized in accumulated other comprehensive loss that have not yet been recognized as components of net periodic pension benefit (expense)
Net actuarial loss, pre-tax	(7,885,000)	(7,704,000)
Components of net periodic pension benefit
Interest cost	873,000	867,000	948,000
Expected return on plan assets	(1,181,000)	(1,137,000)	(1,158,000)
Recognized net actuarial loss	161,000	136,000	146,000
Total net periodic pension cost	(147,000)	$ (134,000)	$ (64,000)
Amounts included in accumulated comprehensive loss which are expected to be recognized as components of net periodic benefit (expense) in next fiscal year
Actuarial loss	$ 163,000
Net Periodic Pension Cost
Weighted-average discount rate (as a percent)	4.30%	4.40%	4.10%
Expected long-term rate of return on plan assets (as a percent)	8.00%	8.00%	8.00%
Benefit Obligation
Weighted-average discount rate (as a percent)	3.90%	4.30%