Equities - Narrative (Details) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jun. 30, 2014 USD ($)	Aug. 31, 2015 USD ($) pools $ / shares shares	Aug. 31, 2014 USD ($)	Aug. 31, 2013 USD ($)	Aug. 31, 2011 USD ($)	Aug. 30, 2014 shares	Sep. 26, 2013 shares
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share | $ / shares		$ 25.00
Patronage Refunds, payable in next fiscal year		$ 625,400
Patronage Refunds		271,226	$ 286,789	$ 380,947
Cash partonage dividends payable		$ 250,200
Patronage source earnings, percentage allocated to reserves		10.00%	10.00%	10.00%
Capital equity certificates, number of pools | pools		2
Equity redemptions, age		70 years
Estimated payments for redemptions of equities		$ (107,300)
Retirements of equities		(128,907)	$ (99,609)	$ (193,413)
Preferred stock dividends paid		(133,710)	(50,761)	$ (24,544)
Preferred stock, Value available for issuance		$ 990,000
Stock Redeemed or Called During Period, Price per Share | $ / shares		$ 29.62
Non-qualified Equity Certificates [Member]
Class of Stock [Line Items]
Patronage Refunds		$ (147,710)	(131,661)
Patronage refunds and capital stock
Class of Stock [Line Items]
Patronage Refunds			821,500	$ 841,100	$ 976,000
Cash Distribution [Member]
Class of Stock [Line Items]
Patronage Refunds		271,200	286,800	380,900
Capital Equity Certificates
Class of Stock [Line Items]
Patronage Refunds		$ (402,560)	(422,670)	$ (595,022)
Patron equities to be redeemed, value			$ 200,000
8% Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage		8.00%
Preferred stock, shares outstanding | shares		12,272,003
Preferred Stock, Redemption Amount		$ 306,800
Proceeds from issuance of preferred stock, net of issuance costs		$ 311,200
Class B, Series 1 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage		7.875%
Preferred stock, shares outstanding | shares		18,071,363
Preferred Stock, Redemption Amount		$ 451,800
Proceeds from issuance of preferred stock, net of issuance costs		$ 472,800
Preferred Stock, Shares Issued | shares						6,752,188	11,319,175
Class B, Series 3 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage		6.75%
Preferred stock, shares outstanding | shares		19,700,000
Preferred Stock, Redemption Amount		$ 492,500
Proceeds from issuance of preferred stock, net of issuance costs		$ 476,700
Class B, Series 2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage		7.10%
Preferred stock, shares outstanding | shares		16,800,000
Preferred Stock, Redemption Amount		$ 420,000
Proceeds from issuance of preferred stock, net of issuance costs		$ 406,200
Class B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, Value registered	$ 2,000,000
London Interbank Offered Rate (LIBOR) [Member] | Class B, Series 3 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent		4.155%
London Interbank Offered Rate (LIBOR) [Member] | Class B, Series 2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent		4.298%
Maximum | Class B, Series 3 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent		8.00%
Maximum | Class B, Series 2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent		8.00%