Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Property, plant and equipment
Less: Accumulated depreciation	$ (200,506)		$ (203,876)
Property, plant and equipment, net	99,878		99,401
Depreciation	8,537	$ 7,537
Land
Property, plant and equipment
Property, plant and equipment gross	1,136		1,136
Buildings and improvements
Property, plant and equipment
Property, plant and equipment gross	73,735		70,522
Machinery and equipment
Property, plant and equipment
Property, plant and equipment gross	$ 225,513		$ 231,619

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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