Accrued Liabilities (Schedule of Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Salaries and wages	$ 23,571	$ 22,878
Contract loss reserve	11,250	17,190
Employee benefits	12,952	14,235
Returns and allowances	10,560	11,249
Accrual for compensated absences	10,655	10,445
Dividends	6,469	6,139
Contract liabilities	8,206	5,656
Lease liability - Operating lease	4,433	4,023
Lease liability - Financing lease	1,438	1,835
Postretirement medical benefits – current portion	3,660	3,808
Restructuring costs	1,908	1,342
Professional fees	3,451	2,999
Pension liability – current portion	2,281	2,155
Workers' compensation	1,685	1,982
Utilities	1,285	790
Interest	701	517
Other	20,954	18,642
Total	$ 125,459	$ 125,885