Notes Payable and Long-Term Debt (Details) (Credit Agreement, USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended
	Aug. 31, 2011	Dec. 31, 2011
Notes Payable and Long-Term Debt
Current borrowing capacity	200,000
Term of agreement (in years)	5
Maximum available for the issuance of letters of credit	50,000
Maximum available for swing loans	5,000
Additional available credit	100,000
Maximum available with contingent increase	300,000
Period of variable interest rate basis (in days)	30
Adjusted LIBOR rate at period end (as a percent)		0.30%
Additional Financial Covenants Required
Outstanding letters of credit		553
Amount available under the Credit Agreement		199,447
Maximum
Notes Payable and Long-Term Debt
Fees on the daily unused amount (as a percent)	0.30%
Additional Financial Covenants Required
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 judgement 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
Minimum
Notes Payable and Long-Term Debt
Fees on the daily unused amount (as a percent)	0.20%
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
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
LIBOR based interest rates
Notes Payable and Long-Term Debt
Variable interest rate basis	adjusted LIBOR
Adjusted LIBOR based interest rates | Maximum
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	1.50%
Adjusted LIBOR based interest rates | Minimum
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	1.25%
Alternate Base Rate based interest rates
Notes Payable and Long-Term Debt
Variable interest rate basis	alternate base rate
Alternate Base Rate based interest rates | Maximum
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	0.50%
Alternate Base Rate based interest rates | Minimum
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	0.25%