Credit Agreement (Details) (Credit Agreement, USD $) In Thousands, unless otherwise specified	1 Months Ended	9 Months Ended			1 Months Ended				1 Months Ended
	Aug. 30, 2012 Y	Sep. 30, 2012	Sep. 30, 2012 Maximum	Sep. 30, 2012 Minimum	Aug. 31, 2012 LIBOR based interest rates day	Sep. 30, 2012 Adjusted LIBOR based interest rates First 30 days Minimum	Sep. 30, 2012 Adjusted LIBOR based interest rates Thereafter Maximum	Sep. 30, 2012 Adjusted LIBOR based interest rates Thereafter Minimum	Aug. 31, 2012 Alternate Base Rate based interest rates	Sep. 30, 2012 Alternate Base Rate based interest rates First 30 days Minimum	Sep. 30, 2012 Alternate Base Rate based interest rates Thereafter Maximum	Sep. 30, 2012 Alternate Base Rate based interest rates Thereafter Minimum
Notes Payable and Long-Term Debt
Term of agreement (in years)	5
Current borrowing capacity	$ 400,000	$ 1,700
Maximum available for the issuance of letters of credit	75,000
Maximum available with contingent increase	500,000
Additional available credit	100,000
Spread on variable interest rate (as a percent)						1.75%	2.25%	1.50%		0.75%	1.25%	0.50%
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)		0.21%
Fees on the daily unused amount (as a percent)		0.25%	0.35%	0.20%
Outstanding borrowings		275,000
Outstanding letters of credit		189
Amount available under the Credit Agreement		124,811
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
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
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
Weighted average interest rate (as a percent)		2.00%
Deferred financing costs		1,700
Line of credit borrowing, subsequent to balance sheet date		32,000
Repayments of debt, subsequent to balance sheet date		7,000
Available borrowing capacity, subsequent to balance sheet date		$ 300,000