Select balance sheet data - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 300,612	$ 283,179
Less accumulated depreciation	185,917	176,491
Total property, plant and equipment, net	114,695	106,688
Land [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 1,033	1,033
Property, plant and equipment useful lives	Indefinite
Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 3,169	3,169
Land Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful lives	15 years
Land Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful lives	39 years
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 55,556	55,172
Building and Building Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful lives	15 years
Building and Building Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful lives	39 years
Machinery, Equipment and Tooling [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 208,170	199,854
Machinery, Equipment and Tooling [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful lives	3 years
Machinery, Equipment and Tooling [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful lives	10 years
Vehicles [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 3,841	3,778
Property, plant and equipment useful lives	5 years
Office Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 17,126	16,242
Office Furniture and Fixtures [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful lives	3 years
Office Furniture and Fixtures [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful lives	7 years
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 11,717	$ 3,931