Fair Value Measurements and Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair value, measurements, recurring [Member] | Quoted prices in active markets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets invested for self-insurance, classified as other assets, noncurrent	$ 14,156	$ 13,064
Interest rate swaps, classified as accrued expenses		0
Fuel derivatives, classified as accrued expenses	0	0
Fuel derivatives, classified as other noncurrent liabilities	0	0
Deferred compensation	0	0
Fair value, measurements, recurring [Member] | Significant other observable inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets invested for self-insurance, classified as other assets, noncurrent	0	0
Interest rate swaps, classified as accrued expenses		123
Fuel derivatives, classified as accrued expenses	306	108
Fuel derivatives, classified as other noncurrent liabilities	237	317
Deferred compensation	787	684
Fair value, measurements, recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets invested for self-insurance, classified as other assets, noncurrent	0	0
Interest rate swaps, classified as accrued expenses		0
Fuel derivatives, classified as accrued expenses	0	0
Fuel derivatives, classified as other noncurrent liabilities	0	0
Deferred compensation	0	0
Fair value, measurements, recurring [Member] | Carrying (reported) amount, fair value disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets invested for self-insurance, classified as other assets, noncurrent	14,156	13,064
Interest rate swaps, classified as accrued expenses		123
Fuel derivatives, classified as accrued expenses	306	108
Fuel derivatives, classified as other noncurrent liabilities	237	317
Deferred compensation	787	684
Carrying (reported) amount, fair value disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps, classified as accrued expenses	0	123
Fuel derivatives, classified as accrued expenses	306	108
Fuel derivatives, classified as other noncurrent liabilities	$ 237	$ 317