Notes Payable and Long-Term Debt (Details) (Credit Agreement, USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended
	Aug. 31, 2012 Y	Dec. 31, 2012
Notes Payable and Long-Term Debt
Term of agreement (in years)	5
Current borrowing capacity	$ 400,000
Maximum available for the issuance of letters of credit	75,000
Maximum available with contingent increase	500,000
Additional available credit	100,000
Maximum available for swing loans	5,000
Fees on the daily unused amount (as a percent)		0.25%
Outstanding borrowings		33,000
Outstanding letters of credit		189
Amount available under the Credit Agreement		366,811
Additional Financial Covenants Required
Weighted average interest rate (as a percent)		2.00%
Deferred financing costs		1,800
Repayment of debt		33,000
Maximum
Notes Payable and Long-Term Debt
Fees on the daily unused amount (as a percent)		0.35%
Additional Financial Covenants Required
Additional secured debt related to a capital asset allowed under Credit Agreement covenants		65,000
Unsecured debt allowed under Credit Agreement covenants for new corporate headquarters		75,000
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
Maximum limit on acquisitions under terms of the Credit Agreement covenants		100,000
Total adjusted leverage ratio, numerator, to allow for maximum limit on acquisitions under terms of the Credit Agreement covenants		2.75
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
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
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 limit on share repurchases for first , second and fourth quarter		150,000
Amount of cash plus unused credit to allow for no limit on share repurchases for third quarter		$ 75,000
Asset coverage ratio, numerator, to be maintained under Credit Agreement covenants on proforma basis		1.1
LIBOR based interest rates
Notes Payable and Long-Term Debt
Variable interest rate basis	adjusted LIBOR
Period of variable interest rate basis (in days)	30
Adjusted LIBOR rate at period end (as a percent)		0.21%
LIBOR based interest rates | First 30 days
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)		1.75%
Adjusted LIBOR based interest rates | Thereafter | Maximum
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)		2.25%
Adjusted LIBOR based interest rates | Thereafter | Minimum
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)		1.50%
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 | First 30 days
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)		0.75%
Alternate Base Rate based interest rates | Thereafter | Maximum
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)		1.25%
Alternate Base Rate based interest rates | Thereafter | Minimum
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)		0.50%