Schedule of Significant Components of Deferred Tax Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Aug. 31, 2012	Aug. 31, 2011
Deferred tax assets:
Alternative minimum tax credit	$ 3,354	$ 3,108
Postretirement benefits	18,060	12,377
Provisions for accrued expenses	2,908	1,893
Stock-based compensation	2,420	2,357
Deferred flood losses		2,625
Net operating loss carryforward	7,469	8,242
Tax credit carryforwards	4,152	4,325
Other	2,025	2,935
Total deferred tax assets gross	40,388	37,862
Less-valuation allowance	(1,787)
Total deferred tax assets	38,601	37,862
Deferred tax liabilities:
Depreciation	21,762	21,678
Hedging	1,004	448
Other	2,560	2,132
Total deferred tax liabilities	25,326	24,258
Net deferred tax assets	13,275	13,604
Recognized as:
Other current assets	167	909
Deferred tax asset	13,108	12,695
Total net deferred tax assets	$ 13,275	$ 13,604