Long-Term Obligations (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	May 26, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Promissory notes	$ 8,200,000		$ 8,200,000			$ 14,400,000
Total debt	153,200,000		153,200,000			181,900,000
Current portion of long-term obligations	(33,525,000)		(33,525,000)			(37,178,000)
Total	119,726,000		119,726,000			144,688,000
Total leverage ratio	0.9		0.9
Fixed charge coverage ratio	1.7		1.7
Amount of revolving credit facility					250,000,000
Availability under the revolving credit facility	231,300,000		231,300,000
Outstanding in letters of credit	18,700,000		18,700,000
Weighted-average interest rate for five year term loan	1.00%	1.10%	1.00%	1.10%
Series A Notes [Member]
Debt Instrument [Line Items]
Senior notes	35,000,000		35,000,000			35,000,000
Principal amount	35,000,000		35,000,000
Senior long-term notes, interest rate	6.07%		6.07%
Maturity date			Mar. 25, 2013
Series B Notes [Member]
Debt Instrument [Line Items]
Senior notes	30,000,000		30,000,000			30,000,000
Principal amount	30,000,000		30,000,000
Senior long-term notes, interest rate	6.28%		6.28%
Maturity date			Mar. 25, 2014
Series C Notes [Member]
Debt Instrument [Line Items]
Senior notes	35,000,000		35,000,000			35,000,000
Principal amount	35,000,000		35,000,000
Senior long-term notes, interest rate	6.49%		6.49%
Maturity date			Mar. 25, 2015
Term Loan [Member]
Debt Instrument [Line Items]
Senior notes	45,000,000		45,000,000			67,500,000
Principal amount	150,000,000		150,000,000
Term loan principal payments plus accrued interest payable, quarterly	$ 7,500,000		$ 7,500,000
Eurodollar Rate plus the applicable percentage			0.99%
Maturity date			Mar. 26, 2013