Property, Plant and Equipment - Summary of property, plant and equipment presented at cost net of accumulated depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 2,110,877	$ 2,088,546
Less: accumulated depreciation	(1,422,542)	(1,239,163)
Net property, plant and equipment	688,335	849,383
Land
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	19,056	18,563
Buildings and leasehold improvements
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	142,715	134,994
Operating equipment
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,440,508	1,425,846
Computer software
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	19,650	19,005
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	8,043	7,835
Vehicles
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	480,899	479,628
Construction in progress
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 6	$ 2,675