Derivative Financial Instruments (Fair Values of Derivative Instruments Designated as Cash Flow Hedges) (Details) - USD ($) $ in Thousands	Jun. 30, 2018		Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, asset derivatives	$ 37,454		$ 24,255
Derivatives designated as cash flow hedges, liability derivatives	(1,678)		(1,396)
Interest Rate Swap [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, liability derivatives			(903)
Interest Rate Swap [Member] | Other Long-term Liabilities [Member}
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, liability derivatives	(1,678)	[1]	(493)
Interest Rate Swap [Member] | Prepaid Expenses and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, asset derivatives	9,208	[1]	5,193
Interest Rate Swap [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivatives designated as cash flow hedges, asset derivatives	24,542		15,182
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	$ 3,704	[2]	$ 3,880

[1] Represents the estimated amount of the existing unrealized gains on interest rate swaps as of June 30, 2018 (based on the interest rate yield curve at that date), included in AOCIL 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 gains on the fuel hedge as of June 30, 2018 (based on the forward DOE diesel fuel index curve at that date), included in AOCIL 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.