Property, Plant and Equipment (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Property, Plant and Equipment
Property, plant and equipment - gross	$ 189,380,000		$ 189,076,000
Less accumulated depreciation and amortization, including accumulated amortization of capitalized leases	95,488,000		92,980,000
Total property, plant and equipment, net	93,892,000		96,096,000
Asset impairment charge	0
Depreciation	2,944,000	$ 3,282,000
Land
Property, Plant and Equipment
Property, plant and equipment - gross	14,881,000		14,869,000
Improvements to land and leaseholds
Property, Plant and Equipment
Property, plant and equipment - gross	17,482,000		17,415,000
Buildings
Property, Plant and Equipment
Property, plant and equipment - gross	34,951,000		34,929,000
Machinery and equipment, including equipment under capitalized leases
Property, Plant and Equipment
Property, plant and equipment - gross	120,636,000		120,806,000
Construction in progress
Property, Plant and Equipment
Property, plant and equipment - gross	$ 1,430,000		$ 1,057,000