Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Measurements, Recurring [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swap derivative instruments - net liability position	$ (11,406)	$ (9,118)
Fuel hedge derivative instruments - net asset position	1,945	3,506
Restricted assets	30,101	30,728
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Assets and liabilities measured at fair value on a recurring basis
Restricted assets	30,101	30,728
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	(11,406)	(9,118)
Significant Unobservable Inputs (Level 3) [Member]
Assets and liabilities measured at fair value on a recurring basis
Fuel hedge derivative instruments - net asset position	$ 1,945	$ 3,506