Property, Plant and Equipment - Summary of property, plant and equipment presented at cost net of accumulated depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 2,151,809	$ 2,103,239
Less: accumulated depreciation	(1,633,827)	(1,659,311)
Net property, plant and equipment	517,982	443,928
Land
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	21,159	19,991
Buildings and leasehold improvements
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	140,269	138,072
Operating equipment
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,429,906	1,419,670
Computer software
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	24,619	23,017
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	7,606	7,656
Vehicles
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 528,250	$ 494,833