Derivative Financial Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative Financial Instruments (Textual) [Abstract]
Number of interest rate swap agreements	3		3
Number of fuel hedge agreements	2		2
Cash Flow Hedging [Member]
Summary of Cash Flow Hedges on results of operations, comprehensive income and accumulated other comprehensive loss
Amount of Gain or (Loss) Recognized as AOCL on Derivatives, Net of Tax (Effective Portion)	$ (2,846)	$ (2,104)	$ (2,555)	$ 819
Amount of (Gain) or Loss Reclassified from AOCL into Earnings, Net of Tax (Effective portion)	162	193	169	848
Interest Rate Swap [Member] | Cash Flow Hedging [Member]
Summary of Cash Flow Hedges on results of operations, comprehensive income and accumulated other comprehensive loss
Amount of Gain or (Loss) Recognized as AOCL on Derivatives, Net of Tax (Effective Portion)	(2,350)	(1,655)	(2,972)	(1,614)
Interest Rate Swap [Member] | Interest Expense [Member]
Summary of Cash Flow Hedges on results of operations, comprehensive income and accumulated other comprehensive loss
Amount of (Gain) or Loss Reclassified from AOCL into Earnings, Net of Tax (Effective portion)	847	985	1,554	2,142
Fuel Hedges [Member]
Summary of Cash Flow Hedges on results of operations, comprehensive income and accumulated other comprehensive loss
Amount of Gain or (Loss) Recognized as AOCL on Derivatives, Net of Tax (Effective Portion)	(496)	(449)	417	2,433
Fuel Hedges [Member] | Cost of Operations [Member]
Summary of Cash Flow Hedges on results of operations, comprehensive income and accumulated other comprehensive loss
Amount of (Gain) or Loss Reclassified from AOCL into Earnings, Net of Tax (Effective portion)	$ (685)	$ (792)	$ (1,385)	$ (1,294)