Fair Values of Derivative Instruments Designated as Cash Flow Hedges (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivatives Designated as cash flow Hedges, Asset derivatives	$ 1,778		$ 1,187
Derivatives Designated as cash flow Hedges, Liability derivatives	(9,823)		(11,163)
Interest Rate Swap | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivatives Designated as cash flow Hedges, Liability derivatives	(5,048)	[1]	(5,374)
Interest Rate Swap | Other Long-term Liabilities
Derivatives, Fair Value [Line Items]
Derivatives Designated as cash flow Hedges, Liability derivatives	(4,775)		(5,789)
Fuel Hedge | Prepaid Expenses and Other Current Assets
Derivatives, Fair Value [Line Items]
Derivatives Designated as cash flow Hedges, Asset derivatives	282	[2]
Fuel Hedge | Other Assets
Derivatives, Fair Value [Line Items]
Derivatives Designated as cash flow Hedges, Asset derivatives	$ 1,496		$ 1,187

[1] Represents the estimated amount of the existing unrealized losses on interest rate swaps as of March 31, 2013 (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 gains on the fuel hedge as of March 31, 2013 (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.