Property, Plant and Equipment - Summary of property, plant and equipment presented at cost net of accumulated depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 2,093,494	$ 2,110,877
Less: accumulated depreciation	(1,595,508)	(1,422,542)
Net property, plant and equipment	497,986	688,335
Land
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	19,070	19,056
Buildings and leasehold improvements
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	142,741	142,715
Operating equipment
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,432,007	1,440,508
Computer software
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	22,050	19,650
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	8,056	8,043
Vehicles
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	469,570	480,899
Construction in progress
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment		$ 6