Long-Term Debt (Details)	12 Months Ended				1 Months Ended				1 Months Ended					1 Months Ended
	Aug. 31, 2012 USD ($)	Aug. 31, 2011 USD ($)	Aug. 31, 2012 For Certain Obligations, Workers Compensation and Performance Bonds [Member] USD ($)	Aug. 31, 2011 For Certain Obligations, Workers Compensation and Performance Bonds [Member] USD ($)	Apr. 30, 2012 Bank of America NA And Other Lenders [Member] USD ($)	Aug. 31, 2012 Bank of America NA And Other Lenders [Member] USD ($)	Apr. 30, 2012 Bank of America NA And Other Lenders [Member] CAD	Aug. 31, 2011 Bank of America NA And Other Lenders [Member] USD ($)	Apr. 30, 2012 Bank of America NA And Other Lenders [Member] Minimum [Member]	Apr. 30, 2012 Bank of America NA And Other Lenders [Member] Maximum [Member]	Apr. 30, 2012 Bank of America NA And Other Lenders [Member] Interest Rate Option 1 [Member]	Apr. 30, 2012 Bank of America NA And Other Lenders [Member] Interest Rate Option 1 [Member] Minimum [Member]	Apr. 30, 2012 Bank of America NA And Other Lenders [Member] Interest Rate Option 1 [Member] Maximum [Member]	Apr. 30, 2012 Bank of America NA And Other Lenders [Member] Interest Rate Option 2 [Member]	Apr. 30, 2012 Bank of America NA And Other Lenders [Member] Interest Rate Option 3 [Member]
Debt Instrument [Line Items]
Bank unsecured revolving credit facility, interest at LIBOR plus a spread						$ 325,292,000		$ 393,428,000
Tax-exempt economic development revenue bonds due January 2021, interest payable monthly at a variable rate (0.23% as of August 31, 2012), secured by a letter of credit	7,700,000	7,700,000
Tax-exempt economic development revenue bonds, variable interest rate	0.23%
Tax Exempt Economic Development Revenue Bond Due Date	2021-01-31
Capital lease obligations due through February 2021	2,320,000	2,802,000
Capital lease obligations due date	2/28/2021
Total long-term debt	335,312,000	403,930,000
Less current maturities	(683,000)	(643,000)
Long-term debt, net of current maturities	334,629,000	403,287,000
Line Of Credit Amendment Date					Apr. 30, 2012
Line of Credit Facility, Increase (Decrease) In Maximum Borrowing Capacity					50,000,000
Line of Credit Facility, Maximum Borrowing Capacity					670,000,000		30,000,000
Debt Instrument, Description of Variable Rate Basis											London Interbank Offered Rate (or the Canadian equivalent)			greater of the prime rate, the federal funds rate	British Bankers Association LIBOR Rate
Debt Instrument, Basis Spread on Variable Rate												1.25%	2.25%	0.50%	1.75%
Line of Credit Facility, Commitment Fee Percentage									0.15%	0.35%
Line of Credit Facility, Expiration Date					April 2017
Line of Credit Facility, Amount Outstanding						325,292,000		393,428,000
Long-term Debt, Weighted Average Interest Rate						2.06%		2.48%
Letters of Credit Outstanding, Amount			$ 18,000,000	$ 18,000,000