Fair Value Measurements - Recurring Fair Value Measurements (Details) - USD ($) $ in Thousands	Aug. 31, 2019	Aug. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative Asset	$ 21,400
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred Compensation Assets, at Fair Value	40,368	$ 39,073
Embedded Derivative, Fair Value of Embedded Derivative Asset	21,364	23,595
Securities Segregated under Other Regulations	77,777
Other assets	6,519	5,334
Total assets	414,417	397,249
Total liabilities	249,461	455,842
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred Compensation Assets, at Fair Value	40,368	39,073
Embedded Derivative, Fair Value of Embedded Derivative Asset	0	0
Securities Segregated under Other Regulations	77,777
Other assets	6,519	5,334
Total assets	192,481	98,894
Total liabilities	40,305	31,778
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred Compensation Assets, at Fair Value	0	0
Embedded Derivative, Fair Value of Embedded Derivative Asset	21,364	23,595
Securities Segregated under Other Regulations	0
Other assets	0	0
Total assets	221,936	298,355
Total liabilities	209,156	424,064
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred Compensation Assets, at Fair Value	0	0
Embedded Derivative, Fair Value of Embedded Derivative Asset	0	0
Securities Segregated under Other Regulations	0
Other assets	0	0
Total assets	0	0
Total liabilities	0	0
Fair Value, Measurements, Recurring | Commodity derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	248,209	313,846
Derivative Liability	228,760	421,689
Fair Value, Measurements, Recurring | Commodity derivatives | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	67,817	54,487
Derivative Liability	40,305	31,778
Fair Value, Measurements, Recurring | Commodity derivatives | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	180,392	259,359
Derivative Liability	188,455	389,911
Fair Value, Measurements, Recurring | Commodity derivatives | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0	0
Derivative Liability	0	0
Fair Value, Measurements, Recurring | Foreign currency derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign Currency Contract, Asset, Fair Value Disclosure	10,339	15,401
Foreign Currency Contracts, Liability, Fair Value Disclosure	20,701	24,701
Fair Value, Measurements, Recurring | Foreign currency derivatives | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign Currency Contract, Asset, Fair Value Disclosure	0	0
Foreign Currency Contracts, Liability, Fair Value Disclosure	0	0
Fair Value, Measurements, Recurring | Foreign currency derivatives | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign Currency Contract, Asset, Fair Value Disclosure	10,339	15,401
Foreign Currency Contracts, Liability, Fair Value Disclosure	20,701	24,701
Fair Value, Measurements, Recurring | Foreign currency derivatives | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign Currency Contract, Asset, Fair Value Disclosure	0	0
Foreign Currency Contracts, Liability, Fair Value Disclosure	0	0
Fair Value, Measurements, Recurring | Interest rate swap derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	9,841
Derivative Liability		9,452
Fair Value, Measurements, Recurring | Interest rate swap derivatives | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0
Derivative Liability		0
Fair Value, Measurements, Recurring | Interest rate swap derivatives | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	9,841
Derivative Liability		9,452
Fair Value, Measurements, Recurring | Interest rate swap derivatives | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	$ 0
Derivative Liability		$ 0