Derivative Financial Instruments (Details 2) (Cash Flow Hedging [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair values of derivative instruments designated as cash flow hedges
Derivatives Designated as cash flow Hedges, Asset derivatives	$ 3,850	$ 3,506
Derivatives Designated as cash flow Hedges, Liability derivatives	(8,981)	(9,118)
Prepaid Expenses and Other Current Assets [Member] | Fuel Hedges [Member]
Fair values of derivative instruments designated as cash flow hedges
Derivatives Designated as cash flow Hedges, Asset derivatives	3,850	3,506
Accrued Liabilities [Member] | Interest Rate Swap [Member]
Fair values of derivative instruments designated as cash flow hedges
Derivatives Designated as cash flow Hedges, Liability derivatives	(5,089)	(4,476)
Other Long Term Liabilities [Member] | Interest Rate Swap [Member]
Fair values of derivative instruments designated as cash flow hedges
Derivatives Designated as cash flow Hedges, Liability derivatives	$ (3,892)	$ (4,642)