Equities - Narrative (Details)	12 Months Ended
	Aug. 31, 2020 USD ($) pools	Aug. 31, 2019 USD ($)	Aug. 31, 2018 USD ($)	Aug. 31, 2017 USD ($)
Class of Stock [Line Items]
Estimated patronage refunds	$ (30,000,000)	$ (90,000,000)	$ (75,000,000)
Cash patronage dividends payable	$ (90,115,000)	(75,776,000)	0
Patronage source earnings, percentage allocated to reserves	10.00%
Capital equity certificates, number of pools | pools	2
Equity redemptions, age	70 years
Redemptions of equities	$ 96,438,000	85,540,000	8,847,000
Preferred stock dividends paid	(168,668,000)	(168,668,000)	(168,668,000)
Reclassifications
Estimated Cash Patronage Dividends Payable	30,000,000.0
futurePaymentsForRepurchaseOfOtherEquity	(33,000,000.0)
Class B, Series 1 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Redemption Amount	536,500,000
Accumulated Other Comprehensive Loss
Class of Stock [Line Items]
Reclassifications	0	(4,706,000)	(26,992,000)
Capital Reserves
Class of Stock [Line Items]
Estimated patronage refunds	241,970,000	562,398,000	420,330,000
Nonqualified Equity Certificates
Class of Stock [Line Items]
Estimated patronage refunds	211,970,000	472,398,000	345,330,000
Qualified Equity Certificates
Class of Stock [Line Items]
Estimated patronage refunds	$ 0
Patronage refunds and capital stock
Class of Stock [Line Items]
PatronageRefunds		564,500,000	428,800,000	$ 128,800,000
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Class of Stock [Line Items]
Reclassifications		$ 983,000	$ (1,468,000)