Equities - Narrative (Details) (USD $)	12 Months Ended
	Aug. 31, 2012 pools	Aug. 31, 2011	Aug. 31, 2010	Aug. 31, 2009
Class of Stock [Line Items]
Net (loss) income	$ 1,335,719,000	$ 1,061,028,000	$ 535,397,000
Dividends and equities payable	578,809,000	400,216,000	210,435,000
Patronage distribution	969,900,000
Cash partonage dividends payable	378,700,000
Cash patronage dividends paid	260,666,000	141,513,000	153,894,000
Patronage source loss				60,200,000
Patronage source earnings, percentage allocated to reserves	10.00%	100.00%
Capital equity certificates, number of pools	2
Equity redemptions, age	70 years
Retirement of equity certificates	196,000,000
Retirements of equities	(145,722,000)	(61,193,000)	(23,135,000)
Capital equity certificates exchanged for preferred stock			142,000
Preferred stock, dividend rate, percentage	8.00%		8.00%
Equity certificate, redemption price per share			$ 28.30
Preferred stock, shares outstanding	12,272,003
Preferred stock, redemption amount	306,800,000
Preferred stock dividends paid	(24,544,000)	(24,544,000)	(23,248,000)
Patronage refunds
Class of Stock [Line Items]
Net (loss) income	969,862,000	674,678,000	396,500,000
Dividends and equities payable	378,719,000	260,125,000	138,775,000
Capital equity certificates exchanged for preferred stock
Patronage refunds and capital stock
Class of Stock [Line Items]
Patronage distribution		676,300,000	402,400,000	438,000,000
Capital equity certificates
Class of Stock [Line Items]
Net (loss) income
Dividends and equities payable	195,999,000	136,000,000	67,569,000
Capital equity certificates exchanged for preferred stock			$ 36,674,000