Derivative Financial Instruments (Details 2) (Cash Flow Hedging [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair values of derivative instruments designated as cash flow hedges
Derivatives Designated as cash flow Hedges, Asset derivatives	$ 1,945	$ 3,506
Derivatives Designated as cash flow Hedges, Liability derivatives	(11,406)	(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	1,458	3,506
Other Assets [Member] | Fuel Hedges [Member]
Fair values of derivative instruments designated as cash flow hedges
Derivatives Designated as cash flow Hedges, Asset derivatives	487
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	(5,084)	(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	$ (6,322)	$ (4,642)