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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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Assets - Property, Plant, and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Property, plant, and equipment
Gross property, plant and equipment	$ 210,798		$ 210,798		$ 204,211
Less: accumulated depreciation and amortization	130,037		130,037		123,927
Net property, plant, and equipment	80,761		80,761		80,284
Depreciation expense	4,300	$ 4,200	8,900	$ 8,400
Land
Property, plant, and equipment
Gross property, plant and equipment	5,669		5,669		5,669
Building and improvements
Property, plant, and equipment
Gross property, plant and equipment	61,407		61,407		61,124
Machinery and equipment
Property, plant, and equipment
Gross property, plant and equipment	136,930		136,930		128,385
Leaseholds improvements
Property, plant, and equipment
Gross property, plant and equipment	$ 6,792		$ 6,792		$ 9,033