Condensed Parent Only Financial Statements - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Cash and cash equivalents	$ 471.3	$ 342.3
Income taxes receivable	5.2	0.3
Financial Instruments, Owned, at Fair Value	2,175.2	2,054.8
Deferred tax assets, Net	18.0	19.8
Property and equipment, net	43.9	42.4
Total assets	9,936.1	7,824.7	$ 6,243.4
Liabilities:
Accounts payable and other accrued liabilities	157.5	145.4
Payables to Customers	3,589.5	3,639.6
Senior secured term loan, net	167.6	0.0			$ 45.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	714.8	866.5
Total liabilities	9,341.9	7,319.4
INTL FCStone Inc. (Parent Company Only) stockholders’ equity:
Common stock, $0.01 par value. Authorized 30,000,000 shares; 21,297,317 issued and 19,075,360 outstanding at September 30, 2019 and 21,030,497 issued and 18,908,540 outstanding at September 30, 2018	0.2	0.2
Common stock in treasury, at cost - 2,221,957 shares at September 30, 2019 and 2,121,957 shares at September 30, 2018	(50.1)	(46.3)
Additional Paid in Capital	276.8	267.5
Retained earnings	402.1	317.0
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	594.2	505.3
Total liabilities and stockholders' equity	9,936.1	7,824.7
Parent Company [Member]
Cash and cash equivalents	2.0	1.8	$ 2.0	$ 1.3
Receivables from subsidiaries	17.6	23.3
Notes receivable, net	2.8	1.8
Income taxes receivable	15.7	14.9
Investment in subsidiaries	399.4	318.0
Financial Instruments, Owned, at Fair Value	0.0	4.4
Deferred tax assets, Net	8.2	8.8
Property and equipment, net	26.9	25.9
Other assets	13.0	7.6
Total assets	486.1	406.5
Liabilities:
Accounts payable and other accrued liabilities	29.4	23.0
Payables to Customers	0.3	1.7
Payables to lenders under loans	70.4	209.4
Senior secured term loan, net	167.6	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	84.5	59.3
Total liabilities	352.2	293.4
INTL FCStone Inc. (Parent Company Only) stockholders’ equity:
Preferred stock, $.01 par value. Authorized 1,000,000 shares; no shares issued or outstanding	0.0	0.0
Common stock, $0.01 par value. Authorized 30,000,000 shares; 21,297,317 issued and 19,075,360 outstanding at September 30, 2019 and 21,030,497 issued and 18,908,540 outstanding at September 30, 2018	0.2	0.2
Common stock in treasury, at cost - 2,221,957 shares at September 30, 2019 and 2,121,957 shares at September 30, 2018	50.1	46.3
Additional Paid in Capital	276.8	267.5
Retained earnings	(93.0)	(108.3)
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	133.9	113.1
Total liabilities and stockholders' equity	486.1	406.5
Adjustment to Investment in Subs for Equity Method Accounting	$ 495.1	$ 425.3