Pensions and Other Postretirement Benefits - Additional Information (Detail) (USD $)	12 Months Ended
	Aug. 31, 2013 Contracts	Aug. 31, 2012	Aug. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Number of defined benefit plans	2
Employees covered under noncontributory defined benefit pension plans (in percentage)	56.00%
Redemption requests notice period (in days)	110 days
Employer contributions	$ 1,100,000	$ 4,100,000	$ 6,400,000
Minimum pension plan funding contribution during fiscal 2014	2,500,000
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return on assets assumption used to calculate net periodic pension expense (in percentage)	7.00%	8.00%	8.00%
Hypothetical decrease in the expected return on assets assumption, to determine impact on expense (in percentage)	0.50%
Estimated increase in expense resulting from a hypothetical decrease of 50 bps in the expected return on plan assets assumption	200,000
Discount rate (in percentage)	4.28%	5.87%	5.64%
Discount rate (in percentage)	5.24%	4.28%	5.87%
Hypothetical decrease in the discount rate assumption, to determine impact on expense (in percentage)	0.25%
Estimated increase in expense resulting from a hypothetical decrease of 25 bps in the discount rate assumption	240,000
Employer contributions	1,136,000	4,052,000
Pension Benefits [Member] | Penford Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets invested	0
Pension Benefits [Member] | Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return on assets assumption used to calculate net periodic pension expense (in percentage)	7.00%
Discount rate (in percentage)	5.24%
Defined Benefit Plan, Amortization of Net Gains (Losses), next fiscal year	500,000
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate (in percentage)	4.28%	5.87%	5.64%
Discount rate (in percentage)	5.24%	4.28%	5.87%
Hypothetical decrease in the discount rate assumption, to determine impact on expense (in percentage)	0.25%
Plan assets invested	0
Employer contributions	$ 551,000	$ 522,000