Derivative Financial Instruments (Fair Values of Derivative Instruments Designated as Cash Flow Hedges) (Details) - USD ($) $ in Thousands	Mar. 31, 2016		Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, liability derivatives	$ (24,129)		$ (19,645)
Interest Rate Swap [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, liability derivatives	(6,128)	[1]	(5,425)
Interest Rate Swap [Member] | Other Long-term Liabilities [Member}
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, liability derivatives	(8,597)		(4,320)
Fuel [Member] | Commodity Contract [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, liability derivatives	(5,836)	[2]	(5,699)
Fuel [Member] | Commodity Contract [Member] | Other Long-term Liabilities [Member}
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, liability derivatives	$ (3,568)		$ (4,201)
Fuel [Member] | Commodity Contract [Member] | Prepaid Expenses and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, asset derivatives

[1] Represents the estimated amount of the existing unrealized losses on interest rate swaps as of March 31, 2016 (based on the interest rate yield curve at that date), included in AOCL expected to be reclassified into pre-tax earnings within the next 12 months. The actual amounts reclassified into earnings are dependent on future movements in interest rates.

[2] Represents the estimated amount of the existing unrealized losses on fuel hedges as of March 31, 2016 (based on the forward DOE diesel fuel index curve at that date), included in AOCL expected to be reclassified into pre-tax earnings within the next 12 months. The actual amounts reclassified into earnings are dependent on future movements in diesel fuel prices.