Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	9 Months Ended
Jun. 30, 2020	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 9.8	$ 306.0
Cash and Securities Segregated under Federal and Other Regulations	1,145.2	1,049.9
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net (including $1261.6 and $626.9 at fair value at June 30, 2020 and September 30, 2019, respectively)	3,396.7	2,540.5
Financial Instruments, Owned, at Fair Value	2,831.9	2,175.2
Physical commodities inventory	270.2	229.3
Accounts Payable and Other Accrued Liabilities	176.4	157.5
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	(430.0)	(266.2)
Financial Instruments Sold, Not yet Purchased, at Fair Value	$ 728.3	714.8
Document Period End Date	Jun. 30, 2020
Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	$ 9.8	306.0
Receivables from Clearing Organizations	1,261.6	626.9
Financial Instruments, Owned, at Fair Value	2,831.9	2,175.2
Assets, Fair Value Disclosure	4,291.0	3,273.8
Financial Instruments Sold, Not yet Purchased, at Fair Value	728.3
Liabilities, Fair Value Disclosure	738.7	722.2
Fair Value, Recurring [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	(7.3)	(5.3)
Fair Value, Recurring [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	(1.5)	(0.3)
Fair Value, Recurring [Member] \| Payable to broker-dealers, clearing organizations and counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	(8.8)	(5.6)
Fair Value, Recurring [Member] \| Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts Payable and Other Accrued Liabilities	1.6	1.8
Fair Value, Recurring [Member] \| Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	173.6	151.9
Fair Value, Recurring [Member] \| Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	19.4	29.1
Fair Value, Recurring [Member] \| US Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	347.8	447.1
Financial Instruments Sold, Not yet Purchased, at Fair Value	6.1	43.8
Fair Value, Recurring [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	372.5	248.7
Financial Instruments Sold, Not yet Purchased, at Fair Value	354.1	272.3
Fair Value, Recurring [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	12.6	0.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	2.2
Fair Value, Recurring [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	92.4	66.5
Fair Value, Recurring [Member] \| Commodities leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	23.2	28.6
Financial Instruments Sold, Not yet Purchased, at Fair Value	2.4	113.7
Fair Value, Recurring [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	56.4	48.4
Fair Value, Recurring [Member] \| Exchange firm common stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	9.8	12.7
Fair Value, Recurring [Member] \| Mutual funds and other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	3.4	0.1
Fair Value, Recurring [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	260.7	168.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	217.6	158.1
Fair Value, Recurring [Member] \| Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	75.4	80.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	26.1	39.2
Fair Value, Recurring [Member] \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	1,558.3	1,045.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	21.1	29.6
Fair Value, Recurring [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	1,148.4
Fair Value, Recurring [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	8.2	9.9
Fair Value, Recurring [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	6.9	8.3
Fair Value, Recurring [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	98.1
Fair Value, Recurring [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	9.6	6.2
Fair Value, Recurring [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.2	299.8
Fair Value, Recurring [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	11.4	8.9
Fair Value, Recurring [Member] \| Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	2.7	4.9
Fair Value, Recurring [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	14.1	13.8
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Receivables from Clearing Organizations	(2,797.8)	(3,161.1)
Financial Instruments, Owned, at Fair Value	(880.0)	(420.8)
Assets, Fair Value Disclosure	(3,677.8)	(3,581.9)
Financial Instruments Sold, Not yet Purchased, at Fair Value	(838.3)	(422.2)
Liabilities, Fair Value Disclosure	(3,785.3)	(3,540.8)
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	5.3	1.5
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	2,941.7	3,117.1
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Payable to broker-dealers, clearing organizations and counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	2,947.0	3,118.6
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts Payable and Other Accrued Liabilities	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| US Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	(880.0)	(420.8)
Financial Instruments Sold, Not yet Purchased, at Fair Value	(838.3)	(422.2)
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Commodities leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Exchange firm common stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Mutual funds and other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	(5.3)
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	(2,792.5)
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	9.8	306.0
Receivables from Clearing Organizations	4,033.5	3,735.0
Financial Instruments, Owned, at Fair Value	703.9	470.9
Assets, Fair Value Disclosure	4,781.1	4,532.8
Financial Instruments Sold, Not yet Purchased, at Fair Value	558.5	419.6
Liabilities, Fair Value Disclosure	3,332.4	3,498.7
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	(2,773.9)	(3,079.1)
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Payable to broker-dealers, clearing organizations and counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	(2,773.9)	(3,079.1)
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts Payable and Other Accrued Liabilities	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	19.8	7.1
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| US Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	372.5	248.7
Financial Instruments Sold, Not yet Purchased, at Fair Value	354.1	272.3
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	12.6	0.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	2.2
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.5	1.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Commodities leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	56.4	48.4
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Exchange firm common stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	9.8	12.7
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Mutual funds and other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	3.4	0.1
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	248.7	159.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	202.2	147.3
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	1,148.4
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	8.2	9.9
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	2,876.9
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	9.6	6.2
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.2	299.8
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	11.4	8.9
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	2.7	4.9
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	14.1	13.8
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Receivables from Clearing Organizations	25.9	53.0
Financial Instruments, Owned, at Fair Value	3,008.0	2,125.1
Assets, Fair Value Disclosure	3,187.7	2,322.9
Financial Instruments Sold, Not yet Purchased, at Fair Value	1,008.1	717.4
Liabilities, Fair Value Disclosure	1,190.0	762.5
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	(12.6)	(6.8)
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	(169.3)	(38.3)
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Payable to broker-dealers, clearing organizations and counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	(181.9)	(45.1)
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts Payable and Other Accrued Liabilities	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	153.8	144.8
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	19.4	29.1
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| US Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	347.8	447.1
Financial Instruments Sold, Not yet Purchased, at Fair Value	6.1	43.8
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	971.9	486.3
Financial Instruments Sold, Not yet Purchased, at Fair Value	937.0	480.3
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Commodities leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	23.2	28.6
Financial Instruments Sold, Not yet Purchased, at Fair Value	2.4	113.7
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Exchange firm common stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Mutual funds and other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	12.0	9.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	15.4	10.8
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	75.4	80.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	26.1	39.2
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	1,558.3	1,045.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	21.1	29.6
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	12.2	9.8
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	13.7
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Receivables from Clearing Organizations	0.0	0.0
Financial Instruments, Owned, at Fair Value	0.0	0.0
Assets, Fair Value Disclosure	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Liabilities, Fair Value Disclosure	1.6	1.8
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Payable to broker-dealers, clearing organizations and counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts Payable and Other Accrued Liabilities	1.6	1.8
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| US Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Commodities leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Exchange firm common stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Mutual funds and other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Commodities warehouse receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 0.0	0.0
TBA and forward settling securities [Member] \| Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations		(1.5)
Debt Security, Government, Non-US [Member] \| Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations		0.0
Derivative [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net (including $1261.6 and $626.9 at fair value at June 30, 2020 and September 30, 2019, respectively)		14.8
Derivative [Member] \| Fair Value, Recurring [Member] \| Counterparty And Cash Collateral Netting Adjustment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations		(3,159.6)
Derivative [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations		3,131.2
Derivative [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations		43.2
Derivative [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($)
$ in Millions

9 Months Ended

Jun. 30, 2020

Sep. 30, 2019

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Cash and Cash Equivalents, Fair Value Disclosure

$ 9.8

$ 306.0

Cash and Securities Segregated under Federal and Other Regulations

1,145.2

1,049.9

Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net (including $1261.6 and $626.9 at fair value at June 30, 2020 and September 30, 2019, respectively)