Other Employee Benefits - Additional Information (Detail) (USD $)	12 Months Ended
	Aug. 31, 2012	Aug. 31, 2011	Aug. 31, 2010
Savings and Stock Ownership Plan [Member]
Employee Benefit Plans [Line Items]
Deferral maximum salary percentage	16.00%
Employer's match of the first 3% of an employee's salary contributed by that employee to the plan	100.00%
Percentage Of Salary under first plan	3.00%
Employer's match of the second 3% of an employee's salary contributed by that employee to the plan	50.00%
Company's matching contributions	$ 1.0	$ 0.9	$ 0.9
Postretirement Health Care And Life Insurance Benefits [Member]
Employee Benefit Plans [Line Items]
Pension and postretirement benefits expense, deferred compensation arrangement	5,100,000	4,500,000	4,700,000
Deferred Compensation Plan [Member]
Employee Benefit Plans [Line Items]
Pension and postretirement benefits expense, deferred compensation arrangement	179,000	197,000	204,000
Supplemental Executive Retirement Plans [Member]
Employee Benefit Plans [Line Items]
Pension and postretirement benefits expense, deferred compensation arrangement	258,000	370,000	343,000
Accrued pension plan obligation	$ 2,700,000	$ 2,700,000