Other Comprehensive Income (Loss) ("OCI") - Components of Accumulated Other Comprehensive Income (loss) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	May 31, 2013	May 31, 2012
Reclassification From Accumulated Other Comprehensive Income Current Period Net Of Tax [Abstract]
Net Unrealized Gains (Losses) on Cash Flow Hedging Instruments, Beginning Balance	$ 1,638	$ 731
Net Unrealized Gains (Losses) on Cash Flow Hedging Instruments, Other comprehensive income (loss), net of taxes	(2,251)	(1,401)
Net Unrealized Gains (Losses) on Cash Flow Hedging Instruments, Ending Balance	(613)	(670)
Gains (Losses) on Postretirement Obligations, Beginning Balance	(17,231)	(8,290)
Gains (Losses) on Postretirement Obligations, Other comprehensive income (loss), net of taxes	1,064
Gains (Losses) on Postretirement Obligations, Ending Balance	(16,167)	(8,290)
Accumulated Other Comprehensive Loss, Beginning Balance	(15,593)	(7,559)
Accumulated Other Comprehensive Loss, Other comprehensive income (loss), net of taxes	(1,187)	(1,401)
Accumulated Other Comprehensive Loss, Ending balance	$ (16,780)	$ (8,960)