Equities - Narrative (Details) (USD $)	12 Months Ended																1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended		3 Months Ended		3 Months Ended
	Aug. 31, 2014 pools	Aug. 31, 2013	Aug. 31, 2012	Aug. 31, 2014 Non-qualified Equity Certificates [Member]	Aug. 31, 2013 Non-qualified Equity Certificates [Member]	Aug. 31, 2012 Non-qualified Equity Certificates [Member]	Aug. 31, 2013 Patronage refunds and capital stock	Aug. 31, 2012 Patronage refunds and capital stock	Aug. 31, 2011 Patronage refunds and capital stock	Aug. 31, 2014 Cash Distribution [Member]	Aug. 31, 2013 Cash Distribution [Member]	Aug. 31, 2012 Cash Distribution [Member]	Aug. 31, 2014 Capital Equity Certificates	Aug. 31, 2013 Capital Equity Certificates	Aug. 31, 2012 Capital Equity Certificates	Aug. 31, 2014 8% Preferred Stock [Member]	Sep. 30, 2013 Class B, Series 1 Preferred Stock [Member]	Aug. 31, 2014 Class B, Series 1 Preferred Stock [Member]	Aug. 21, 2014 Class B, Series 1 Preferred Stock [Member]	Sep. 13, 2013 Class B, Series 1 Preferred Stock [Member]	Mar. 31, 2014 Class B, Series 2 Preferred Stock [Member]	Aug. 31, 2014 Class B, Series 2 Preferred Stock [Member]	Jun. 30, 2014 Class B Preferred Stock [Member]	Aug. 31, 2014 London Interbank Offered Rate (LIBOR) [Member] Class B, Series 2 Preferred Stock [Member]	Aug. 31, 2014 Maximum Class B, Series 2 Preferred Stock [Member]	Nov. 30, 2014 Subsequent Event [Member] Class B, Series 3 Preferred Stock [Member]	Sep. 30, 2014 Subsequent Event [Member] Class B, Series 3 Preferred Stock [Member]	Nov. 30, 2014 Subsequent Event [Member] London Interbank Offered Rate (LIBOR) [Member] Class B, Series 3 Preferred Stock [Member]	Nov. 30, 2014 Subsequent Event [Member] Maximum Class B, Series 3 Preferred Stock [Member]
Class of Stock [Line Items]
Patronage Refunds, payable in next fiscal year	$ 662,100,000
Patronage Refunds	286,789,000	380,947,000	260,666,000	(131,661,000)			841,100,000	976,000,000	676,300,000	286,800,000	380,900,000	260,700,000	(422,670,000)	(595,022,000)	(415,584,000)
Patronage distribution				148,600,000
Cash partonage dividends payable	264,800,000
Patronage source earnings, percentage allocated to reserves	10.00%		10.00%
Capital equity certificates, number of pools	2
Equity redemptions, age	70 years
Estimated payments for redemptions of equities	(130,100,000)
Retirements of equities	(99,609,000)	(193,413,000)	(145,722,000)
Preferred stock, dividend rate, percentage																8.00%		7.875%				7.10%				6.75%
Preferred stock, shares outstanding																12,272,003		18,071,363
Preferred stock, redemption amount																306,800,000		451,800,000				420,000,000					492,500,000
Proceeds from issuance of preferred stock, net of issuance costs																	273,300,000				406,300,000					477,000,000
Preferred stock dividends paid	(50,761,000)	(24,544,000)	(24,544,000)
Preferred stock, Value registered																							2,000,000,000
Preferred Stock, Shares Issued																			6,752,188	11,319,175	16,800,000						19,700,000
Preferred Stock, Basis Spread on Dividends, Percent																								4.298%	8.00%			4.155%	8.00%
Patron equities to be redeemed, value													$ 200,000,000
Stock Redeemed or Called During Period, Price per Share	$ 29.62