Long-Term Obligations (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Promissory notes	$ 10,800,000		$ 10,800,000		$ 14,400,000
Total debt	163,300,000		163,300,000		181,900,000
Current portion of long-term obligations	(35,448,000)		(35,448,000)		(37,178,000)
Total	127,901,000		127,901,000		144,688,000
Total leverage ratio	0.9		0.9
Fixed charge coverage ratio	1.5		1.5
Amount of revolving credit facility	250,000,000		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]
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]
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]
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]
Senior notes	52,500,000		52,500,000		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			0.94%
Maturity date			Mar. 26, 2013