Fair Value Measurements - Fair Value Measurements (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Feb. 28, 2021	Aug. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation assets	$ 49,314	$ 47,669
Embedded derivative asset	15,865	18,998
Securities Segregated under Other Regulations	185,224	85,950
Other assets	5,991	5,276
Total assets	1,254,845	530,961
Foreign exchange derivatives		69,467
Total liabilities	580,826	421,630
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	49,314	47,669
Embedded derivative asset	0	0
Securities Segregated under Other Regulations	185,224	85,950
Other assets	5,991	5,276
Total assets	242,385	144,657
Foreign exchange derivatives		0
Total liabilities	4,409	6,037
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation assets	0	0
Embedded derivative asset	15,865	18,998
Securities Segregated under Other Regulations	0	0
Other assets	0	0
Total assets	1,012,460	386,304
Foreign exchange derivatives		69,467
Total liabilities	576,417	415,593
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation assets	0	0
Embedded derivative asset	0	0
Securities Segregated under Other Regulations	0	0
Other assets	0	0
Total assets	0	0
Foreign exchange derivatives		0
Total liabilities	0	0
Commodity derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	991,149	361,545
Derivative Liability	516,157	352,163
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	1,856	5,762
Derivative Liability	4,409	6,037
Commodity derivatives | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	989,293	355,783
Derivative Liability	511,748	346,126
Commodity derivatives | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0	0
Derivative Liability	0	0
Foreign exchange derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange derivatives	7,302	11,523
Foreign exchange derivatives	64,669
Foreign exchange 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 exchange derivatives	0	0
Foreign exchange derivatives	0
Foreign exchange derivatives | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange derivatives	7,302	11,523
Foreign exchange derivatives	64,669
Foreign exchange derivatives | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign exchange derivatives	0	$ 0
Foreign exchange derivatives	$ 0