Credit Agreement (Details) (USD $) In Thousands, unless otherwise specified		1 Months Ended				1 Months Ended	9 Months Ended						9 Months Ended
	Aug. 10, 2011 Previous Credit Agreement	Aug. 31, 2011 Amendment Number Two	Aug. 10, 2011 Amendment Number Two	Sep. 30, 2011 Amendment Number Two LIBOR based interest rates	Sep. 30, 2011 Amendment Number Two Prime Rate based interest rates	Aug. 31, 2011 Credit Agreement. D Y	Sep. 30, 2011 Credit Agreement.	Aug. 30, 2011 Credit Agreement.	Sep. 30, 2011 Credit Agreement. Adjusted LIBOR based interest rates Maximum	Sep. 30, 2011 Credit Agreement. Adjusted LIBOR based interest rates Minimum	Sep. 30, 2011 Credit Agreement. Alternate Base Rate based interest rates Maximum	Sep. 30, 2011 Credit Agreement. Alternate Base Rate based interest rates Minimum	Sep. 30, 2011 Credit Agreement. Maximum	Sep. 30, 2011 Credit Agreement. Minimum
Credit Agreement
Spread on variable interest rate (as a percent)				1.25%	0.25%				1.50%	1.25%	0.50%	0.25%
Current borrowing capacity	$ 20,000		$ 60,000					$ 200,000
Maximum available for the issuance of letters of credit								50,000
Maximum available with contingent increase								300,000
Additional available credit								100,000
Maximum available for swingline loans								5,000
Fees on the daily unused amount (as a percent)													0.30%	0.20%
Term of agreement (in years)						5
Period of variable interest rate basis (in days)						30
Adjusted LIBOR rate at period end (as a percent)							0.24%
Outstanding letters of credit							189
Amount available under the Credit Agreement							154,811
Outstanding borrowings							45,000
Outstanding borrowings repaid subsequent to the end of the reporting period							20,000
Additional Financial Covenants Required
Percentage fee payable for certain letters of credit before amendment (as a percent)		0.75%
Percentage fee payable for certain letters of credit after amendment (as a percent)		1.25%
Asset coverage ratio, numerator, to be maintained under Credit Agreement covenants														1.10
Asset coverage ratio, denominator, to be maintained under Credit Agreement covenants														1.00
Ratio of consolidated EBITDA plus annual rental expense to annual interest expense plus annual rental expense, numerator, to be maintained under Credit Agreement covenants														2.25
Ratio of consolidated EBITDA plus annual rental expense to annual interest expense plus annual rental expense, denominator, to be maintained under Credit Agreement covenants														1.00
Additional secured debt related to a capital asset allowed under Credit Agreement covenants													20,000
Additional unsecured debt allowed under Credit Agreement covenants													200,000
Secured debt not related to a capital asset allowed under Credit Agreement covenants													5,000
Amount of judgment allowed under Credit Agreement covenants													10,000
Amount of ERISA event in one year allowed under Credit Agreement covenants													10,000
Amount of ERISA event in all years allowed under Credit Agreement covenants													20,000
Total adjusted leverage ratio, numerator, to allow for no limit on acquisitions under terms of the Credit Agreement covenants													2.75
Total adjusted leverage ratio, denominator, to allow for no limit on acquisitions under terms of the Credit Agreement covenants													1.00
Amount of cash and unused credit to allow for no limit on acquisitions														75,000
Amount of cash and unused credit to allow for no restrictions on dividends or share repurchases														$ 75,000