Property, Plant and Equipment - Summary of property, plant and equipment are presented at cost net of accumulated depreciation (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 1,795,628	$ 1,680,561
Less: accumulated depreciation	(1,069,321)	(924,235)
Net property, plant and equipment	726,307	756,326
Land
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	19,264	17,420
Buildings and leasehold improvements
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	130,072	111,986
Operating equipment
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,231,504	1,155,600
Computer software
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	17,121	20,581
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	7,737	7,232
Vehicles
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	387,854	357,913
Construction in progress
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 2,076	$ 9,829