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, 2019	Aug. 31, 2018
Reconciliation of gross and net fair value of assets and liabilities subject to offsetting arrangements [Line Items]
Gross Amounts Recognized	$ 246,728	$ 352,029
Derivative asset, fair value, gross amount not offset on Balance sheet	65,834	35,484
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	180,894	316,545
Derivative Liability, Fair Value, Gross Liability	244,019	445,755
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	4,191	12,983
Derivative liability, fair value, gross amount not offset on balance sheet	48,755	35,484
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	191,073	397,288
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	215,030	313,033
Derivative asset, fair value, gross amount not offset on Balance sheet	58,726	26,781
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	156,304	286,252
Derivative Liability, Fair Value, Gross Liability	223,410	421,054
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	4,191	12,983
Derivative liability, fair value, gross amount not offset on balance sheet	41,647	26,781
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	177,572	381,290
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	10,334	15,401
Derivative asset, fair value, gross amount not offset on Balance sheet	7,108	8,703
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	3,226	6,698
Derivative Liability, Fair Value, Gross Liability	20,609	24,701
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0	0
Derivative liability, fair value, gross amount not offset on balance sheet	7,108	8,703
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	13,501	15,998
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	21,364	23,595
Derivative asset, fair value, gross amount not offset on Balance sheet	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	$ 21,364	$ 23,595