Condensed Parent Only Financial Statements - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Cash and cash equivalents	$ 342.3	$ 314.9	$ 316.2	$ 268.1
Income taxes receivable	0.3	0.4
Financial instruments owned, at fair value	2,054.8	1,731.8
Deferred tax assets, Net	19.8	42.6
Property and equipment, net	42.4	38.7
Total assets	7,824.7	6,243.4	5,950.3
Liabilities:
Accounts payable and other accrued liabilities	145.4	135.6
Payables to Customers	3,639.6	3,072.9
Payables to lenders under loans	355.2	230.2
Financial Instruments Sold, Not yet Purchased, at Fair Value	866.5	717.6
Total liabilities	7,319.4	5,793.5
INTL FCStone Inc. (Parent Company Only) stockholders’ equity:
Common stock, $0.01 par value. Authorized 30,000,000 shares; 21,030,497 issued and 18,908,540 outstanding at September 30, 2018 and 20,855,243 issued and 18,733,286 outstanding at September 30, 2017	0.2	0.2
Common stock in treasury, at cost - 2,121,957 shares at September 30, 2018 and 2017	(46.3)	(46.3)
Additional Paid in Capital	267.5	259.0
Retained earnings	317.0	261.5
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	505.3	449.9
Total liabilities and stockholders' equity	7,824.7	6,243.4
Parent Company [Member]
Cash and cash equivalents	1.8	2.0	1.3	$ 2.5
Receivables from subsidiaries	23.3	3.8
Notes receivable, net	1.8	4.8
Income taxes receivable	14.9	8.6
Investment in subsidiaries	318.0	312.3
Financial instruments owned, at fair value	4.4	0.0
Deferred tax assets, Net	8.8	26.5
Property and equipment, net	25.9	24.8
Other assets	7.6	7.6
Total assets	406.5	390.4
Liabilities:
Accounts payable and other accrued liabilities	23.0	19.8
Payables to Customers	1.7	2.1
Payables to lenders under loans	209.4	152.0
Payable to subsidiaries, net	0.0	49.4
Financial Instruments Sold, Not yet Purchased, at Fair Value	59.3	25.3
Total liabilities	293.4	248.6
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; 21,030,497 issued and 18,908,540 outstanding at September 30, 2018 and 20,855,243 issued and 18,733,286 outstanding at September 30, 2017	0.2	0.2
Common stock in treasury, at cost - 2,121,957 shares at September 30, 2018 and 2017	(46.3)	(46.3)
Additional Paid in Capital	267.5	259.0
Retained earnings	(108.3)	(71.1)
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	113.1	141.8
Total liabilities and stockholders' equity	$ 406.5	390.4
Adjustment to Investment in Subs for Equity Method Accounting		$ 425.3	$ 332.6