Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment
Property, plant and equipment, gross	$ 274,440	$ 292,260
Less: accumulated depreciation	(186,615)	(196,973)
Property, plant and equipment net book value of depreciable assets	87,825	95,287
Property and equipment, net of depreciation	125,497	132,348
Automobiles and trucks
Property, Plant and Equipment
Property, plant and equipment, gross	2,379	2,161
Building and improvements
Property, Plant and Equipment
Property, plant and equipment, gross	44,324	44,278
Construction equipment
Property, Plant and Equipment
Property, plant and equipment, gross	142,661	153,147
Vessels and other equipment
Property, Plant and Equipment
Property, plant and equipment, gross	79,499	84,022
Office equipment
Property, Plant and Equipment
Property, plant and equipment, gross	5,577	8,652
Construction in progress
Property, Plant and Equipment
Property, plant and equipment, gross	1,809	1,198
Land
Property, Plant and Equipment
Property, plant and equipment, gross	$ 35,863	$ 35,863

Property, Plant and Equipment, Net Book Value of Depreciable Assets

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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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