Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment
Property, plant and equipment, gross	$ 269,165	$ 274,440
Less: accumulated depreciation	(185,883)	(186,615)
Property, plant and equipment net book value of depreciable assets	83,282	87,825
Property and equipment, net of depreciation	120,879	125,497
Automobiles and trucks
Property, Plant and Equipment
Property, plant and equipment, gross	2,722	2,379
Building and improvements
Property, Plant and Equipment
Property, plant and equipment, gross	44,325	44,324
Construction equipment
Property, Plant and Equipment
Property, plant and equipment, gross	135,485	142,661
Vessels and other equipment
Property, Plant and Equipment
Property, plant and equipment, gross	80,686	79,499
Office equipment
Property, Plant and Equipment
Property, plant and equipment, gross	5,947	5,577
Construction in progress
Property, Plant and Equipment
Property, plant and equipment, gross	1,734	1,809
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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