Fair Value Measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Assets and liabilities measured at fair value on a recurring basis
Interest rate swap derivative instruments - net liability position	$ (9,766)	$ (9,722)
Fuel hedge derivative instruments - net asset position	4,423	4,730
Restricted assets	27,206	30,791
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swap derivative instruments - net liability position	0	0
Fuel hedge derivative instruments - net asset position	0	0
Restricted assets	27,206	30,791
Significant Other Observable Inputs (Level 2) [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swap derivative instruments - net liability position	(9,766)	(9,722)
Fuel hedge derivative instruments - net asset position	0	0
Restricted assets	0	0
Significant Unobservable Inputs (Level 3) [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swap derivative instruments - net liability position	0	0
Fuel hedge derivative instruments - net asset position	4,423	4,730
Restricted assets	$ 0	$ 0