Select balance sheet data - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Total property, plant and equipment, gross	$ 281,048	$ 266,399
Less accumulated depreciation	153,327	142,516
Total property, plant and equipment, net	127,721	123,883
Land [Member]
Total property, plant and equipment, gross	$ 1,264	1,264
Property, plant and equipment useful lives	Indefinite
Land Improvements [Member]
Total property, plant and equipment, gross	$ 3,051	3,169
Land Improvements [Member] | Maximum [Member]
Property, plant and equipment useful lives	39 years
Land Improvements [Member] | Minimum [Member]
Property, plant and equipment useful lives	15 years
Building Improvements [Member]
Total property, plant and equipment, gross	$ 56,451	55,269
Building Improvements [Member] | Maximum [Member]
Property, plant and equipment useful lives	39 years
Building Improvements [Member] | Minimum [Member]
Property, plant and equipment useful lives	15 years
Machinery and Equipment [Member]
Total property, plant and equipment, gross	$ 191,299	182,045
Machinery and Equipment [Member] | Maximum [Member]
Property, plant and equipment useful lives	10 years
Machinery and Equipment [Member] | Minimum [Member]
Property, plant and equipment useful lives	3 years
Vehicles [Member]
Total property, plant and equipment, gross	$ 3,636	3,613
Property, plant and equipment useful lives	5 years
Office Equipment [Member]
Total property, plant and equipment, gross	$ 14,530	14,253
Office Equipment [Member] | Maximum [Member]
Property, plant and equipment useful lives	7 years
Office Equipment [Member] | Minimum [Member]
Property, plant and equipment useful lives	3 years
Construction in Progress [Member]
Total property, plant and equipment, gross	$ 10,817	$ 6,786