Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment
Gross property, plant and equipment	$ 1,913,635	$ 2,151,809
Less: accumulated depreciation	(1,396,908)	(1,633,827)
Net property, plant and equipment	516,727	517,982
Land
Property, Plant and Equipment
Gross property, plant and equipment	18,945	21,159
Building and leasehold improvements
Property, Plant and Equipment
Gross property, plant and equipment	114,318	140,269
Operating equipment
Property, Plant and Equipment
Gross property, plant and equipment	1,194,071	1,429,906
Computer software
Property, Plant and Equipment
Gross property, plant and equipment	25,247	24,619
Furniture and fixtures
Property, Plant and Equipment
Gross property, plant and equipment	7,086	7,606
Vehicles
Property, Plant and Equipment
Gross property, plant and equipment	$ 553,968	$ 528,250

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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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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