Derivative Financial Instruments and Hedging Activities - Gross Fair Values of Derivative Assets, Derivative Liabilities, and Margin Deposits (Cash Collateral) (Details) - USD ($) $ in Thousands	Aug. 31, 2020	Aug. 31, 2019
Reconciliation of gross and net fair value of assets and liabilities subject to offsetting arrangements [Line Items]
Gross Amounts Recognized	$ 358,300	$ 246,728
Derivative asset, fair value, gross amount not offset on Balance sheet	12,365	65,834
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	345,935	180,894
Derivative Liability, Fair Value, Gross Liability	412,809	244,019
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	956	4,191
Derivative liability, fair value, gross amount not offset on balance sheet	14,963	48,755
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	396,890	191,073
Not Designated as Hedging Instrument | Commodity derivatives
Reconciliation of gross and net fair value of assets and liabilities subject to offsetting arrangements [Line Items]
Gross Amounts Recognized	327,493	215,030
Derivative asset, fair value, gross amount not offset on Balance sheet	2,980	58,726
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	324,513	156,304
Derivative Liability, Fair Value, Gross Liability	343,343	223,410
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	956	4,191
Derivative liability, fair value, gross amount not offset on balance sheet	5,578	41,647
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	336,809	177,572
Not Designated as Hedging Instrument | Foreign exchange derivatives
Reconciliation of gross and net fair value of assets and liabilities subject to offsetting arrangements [Line Items]
Gross Amounts Recognized	11,809	10,334
Derivative asset, fair value, gross amount not offset on Balance sheet	9,385	7,108
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	2,424	3,226
Derivative Liability, Fair Value, Gross Liability	69,466	20,609
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0	0
Derivative liability, fair value, gross amount not offset on balance sheet	9,385	7,108
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	60,081	13,501
Not Designated as Hedging Instrument | Embedded derivative asset
Reconciliation of gross and net fair value of assets and liabilities subject to offsetting arrangements [Line Items]
Gross Amounts Recognized	18,998	21,364
Derivative asset, fair value, gross amount not offset on Balance sheet	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	$ 18,998	$ 21,364