Equities - Narrative (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 30, 2014 USD ($)	May 31, 2016 USD ($) $ / shares	Aug. 31, 2016 USD ($) pools $ / shares shares	Aug. 31, 2015 USD ($)	Aug. 31, 2014 USD ($) $ / shares	Aug. 31, 2013 USD ($)	Mar. 31, 2016 USD ($) shares	Aug. 30, 2014 shares	Aug. 25, 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			$ 279,000
Distribution of patronage refunds			251,740	$ 271,226	$ 286,789
Cash partonage dividends payable			$ 111,600	$ 251,700	$ 271,200	$ 286,800
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
Retirements of equities			$ (23,911)	$ (128,907)	$ (99,609)
Preferred stock dividends paid			(163,324)	(133,710)	$ (50,761)
Preferred stock, Value available for issuance			990,000
Stock Redeemed or Called During Period, Price per Share | $ / shares		$ 28.50			$ 29.62
Non-qualified Equity Certificates [Member]
Class of Stock [Line Items]
Distribution of patronage refunds				147,710	$ 131,661
Patronage refunds and capital stock
Class of Stock [Line Items]
Distribution of patronage refunds				627,200	821,500	$ 841,100
Capital Equity Certificates
Class of Stock [Line Items]
Distribution of patronage refunds			$ 375,506	$ 402,560	422,670
Stock Redeemed or Called During Period, Value		$ 76,800			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				$ 67,300
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			20,764,558
Preferred Stock, Redemption Amount			$ 519,100
Proceeds from issuance of preferred stock, net of issuance costs			$ 549,400
Preferred Stock, Shares Issued | shares							2,693,195	6,752,188	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%
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Class of Stock [Line Items]
Loss on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring			$ 3,700
Gain on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring					$ 13,500
Fiscal Year [Domain]
Class of Stock [Line Items]
Estimated payments for redemptions of equities			40,000
Fiscal Year 2015 [Member]
Class of Stock [Line Items]
Estimated payments for redemptions of equities			$ 18,600