Property, Plant and Equipment - Summary of property, plant and equipment presented at cost net of accumulated depreciation (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 2,088,546	$ 1,795,628
Less: accumulated depreciation	(1,239,163)	(1,069,321)
Net property, plant and equipment	849,383	726,307
Land
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	18,563	19,264
Buildings and leasehold improvements
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	134,994	130,072
Operating equipment
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,425,846	1,231,504
Computer software
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	19,005	17,121
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	7,835	7,737
Vehicles
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	479,628	387,854
Construction in progress
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 2,675	$ 2,076