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, 2012	Dec. 31, 2011
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 1,680,561	$ 1,442,626
Less: accumulated depreciation	(924,235)	(767,266)
Net property, plant and equipment	756,326	675,360
Land
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	17,420	17,403
Buildings and leasehold improvements
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	111,986	94,152
Operating equipment
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,155,600	990,186
Computer software
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	20,581	18,662
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	7,232	5,679
Vehicles
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	357,913	306,716
Construction in progress
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 9,829	$ 9,828