Equities - Narrative (Details)		3 Months Ended		12 Months Ended
	Mar. 30, 2017 USD ($) $ / shares shares	Aug. 31, 2018 USD ($) pools	May 31, 2016 USD ($) $ / shares	Aug. 31, 2018 USD ($) pools	Aug. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Aug. 31, 2014 USD ($)	Mar. 31, 2016 USD ($) shares
Class of Stock [Line Items]
Estimated patronage refunds				$ 75,000,000		$ 103,879,000
Cash patronage dividends payable				$ 75,000,000	$ 0	103,900,000	$ 251,700,000
Distribution of patronage refunds					$ 103,879,000	$ 251,740,000
Patronage source earnings, percentage allocated to reserves				10.00%	10.00%	10.00%
Capital equity certificates, number of pools | pools		2		2
Equity redemptions, age				70 years
Redemptions of equities				$ (8,847,000)	$ (35,268,000)	$ (23,911,000)
Stock Redeemed or Called During Period, Price per Share | $ / shares	$ 28.74		$ 28.50
Preferred stock dividends paid				(168,668,000)	(167,642,000)	(163,324,000)
Reclassification of tax effects to retained earnings				0
Class B, Series 1 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued | shares	695,390							2,693,195
Preferred Stock, Redemption Amount		$ 536,500,000		536,500,000
Fiscal Year 2017 [Domain]
Class of Stock [Line Items]
Estimated payments for redemptions of equities				75,000,000
Accumulated Other Comprehensive Loss
Class of Stock [Line Items]
Reclassification of tax effects to retained earnings		(27,000,000)		(26,992,000)
Capital Reserves
Class of Stock [Line Items]
Estimated patronage refunds				420,330,000	126,333,000	257,458,000
Distribution of patronage refunds				128,831,000	257,468,000	627,246,000
Reclassification of tax effects to retained earnings		$ 27,000,000		26,992,000
Nonqualified Equity Certificates
Class of Stock [Line Items]
Estimated patronage refunds				(345,330,000)	(126,333,000)
Distribution of patronage refunds				(128,831,000)
Qualified Equity Certificates
Class of Stock [Line Items]
Estimated patronage refunds				0
Patronage refunds and capital stock
Class of Stock [Line Items]
Distribution of patronage refunds					128,800,000	257,500,000	$ 627,200,000
Capital Equity Certificates
Class of Stock [Line Items]
Estimated patronage refunds						(153,579,000)
Distribution of patronage refunds					$ (153,589,000)	(375,506,000)
Stock Redeemed or Called During Period, Value	$ 20,000,000		$ 76,800,000
Preferred Stock, Redemption Amount	$ 17,400,000							$ 67,300,000
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Class of Stock [Line Items]
Reclassification of tax effects to retained earnings				$ (1,468,000)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Class of Stock [Line Items]
Loss reclassified from accumulated other comprehensive loss						$ 3,700,000