Property, Plant and Equipment - Summary of property, plant and equipment presented at cost net of accumulated depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 2,103,239	$ 2,093,494
Less: accumulated depreciation	(1,659,311)	(1,595,508)
Net property, plant and equipment	443,928	497,986
Land
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	19,991	19,070
Buildings and leasehold improvements
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	138,072	142,741
Operating equipment
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,419,670	1,432,007
Computer software
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	23,017	22,050
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	7,656	8,056
Vehicles
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	494,833	469,570
Construction in progress
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 0	$ 0