Credit Agreement (Details) (Credit Agreement, USD $) In Thousands, unless otherwise specified	1 Months Ended			6 Months Ended		1 Months Ended			1 Months Ended
	Aug. 31, 2011 day Y	Jun. 30, 2012	Aug. 30, 2011	Jun. 30, 2012 Maximum	Jun. 30, 2012 Minimum	Aug. 31, 2011 LIBOR based interest rates	Jun. 30, 2012 Adjusted LIBOR based interest rates Maximum	Jun. 30, 2012 Adjusted LIBOR based interest rates Minimum	Aug. 31, 2011 Alternate Base Rate based interest rates	Jun. 30, 2012 Alternate Base Rate based interest rates Maximum	Jun. 30, 2012 Alternate Base Rate based interest rates Minimum
Notes Payable and Long-Term Debt
Term of agreement (in years)	5
Current borrowing capacity			$ 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
Spread on variable interest rate (as a percent)							1.50%	1.25%		0.50%	0.25%
Variable interest rate basis						adjusted LIBOR			alternate base rate
Maximum available for swing loans			5,000
Period of variable interest rate basis (in days)	30
Adjusted LIBOR rate at period end (as a percent)		0.25%
Fees on the daily unused amount (as a percent)				0.30%	0.20%
Outstanding letters of credit		189
Amount available under the Credit Agreement		199,811
Additional Financial Covenants Required
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 plus unused credit to allow for no limit on acquisitions					75,000
Amount of cash plus unused credit to allow for no restrictions on dividends or share repurchases					75,000
Outstanding borrowings		$ 102,000