Credit Facilities - Credit Facilities and Financing Bridge Commitment (Details)	1 Months Ended
	Feb. 29, 2020 USD ($)	Sep. 30, 2021 USD ($) facility	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Line of Credit Facility [Line Items]
Senior secured bridge loan	$ 350,000,000
Non-refundable commitment fee	$ 4,400,000
Maximum borrowing capacity under credit facilities		$ 806,400,000
Amount outstanding		385,100,000		$ 427,600,000
Payable to lenders under loans		248,600,000		268,100,000
Notes payable to bank		8,600,000	$ 9,000,000	0
Senior Secured Notes		336,900,000		336,000,000.0
Lenders under loans		755,600,000		783,600,000
Debt instrument, unamortized discount (premium) and debt issuance costs, net		$ 200,000
Number of credit facilities scheduled to expire within one year | facility		3
StoneX Inc. Bank of America Syndicated Credit Facility
Line of Credit Facility [Line Items]
Long-term debt, gross		$ 170,300,000
Line of credit facility borrowing		236,100,000		196,500,000
Maximum borrowing capacity under credit facilities		406,400,000
Senior secured borrowings, net		170,100,000		179,500,000
Long-term line of credit, noncurrent		0		23,000,000.0
Long-term debt		170,100,000		202,500,000
StoneX Financial Margin line of credit facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing		75,000,000.0
Line of credit facility		0		0
Payable to lenders under loans		0		0
FMS Sub-note commodity line of credit facility
Line of Credit Facility [Line Items]
Line of credit facility borrowing		300,000,000.0
Line of credit facility		215,000,000.0		200,100,000
StoneX, Ltd
Line of Credit Facility [Line Items]
Line of credit facility borrowing		25,000,000
Line of credit facility		0		25,000,000.0
IFFI Uncommitted Lines of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing		100,000,000
Payable to lenders under loans		0		0
Short-term debt				0
Rabobank Uncommited Line of Credit
Line of Credit Facility [Line Items]
Short-term debt		25,000,000.0		$ 20,000,000.0
Senior Notes
Line of Credit Facility [Line Items]
Debt instrument, unamortized discount (premium) and debt issuance costs, net		$ 11,500,000