Condensed Parent Only Financial Statements - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Cash and cash equivalents	$ 268.1	$ 231.3	$ 156.1	$ 236.3
Income taxes receivable	10.6	10.8
Financial instruments owned, at fair value	1,421.9	197.9
Deferred tax assets, Net	28.2	32.0
Property and equipment, net	19.7	15.9
Goodwill and intangible assets, net	58.1	58.0
Total assets	5,070.0	3,039.7	2,848.0
Liabilities:
Accounts payable and other accrued liabilities	144.8	114.1
Payables to Customers	2,593.5	2,228.7
Payables to lenders under loans	41.6	22.5
Senior unsecured notes	45.5	45.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	568.3	264.0
Total liabilities	4,672.9	2,694.3
INTL FCStone Inc. (Parent Company Only) stockholders’ equity:
Common stock, $0.01 par value. Authorized 30,000,000 shares; 20,184,556 issued and 18,812,803 outstanding at September 30, 2015 and 19,826,635 issued and 18,883,662 outstanding at September 30, 2014	0.2	0.2
Common stock in treasury, at cost - 1,371,753 shares at September 30, 2015 and 942,973 shares at September 30, 2014	(26.8)	(17.5)
Additional Paid in Capital	240.8	229.6
Retained earnings	200.4	144.7
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	397.1	345.4
Total liabilities and stockholders' equity	5,070.0	3,039.7
Parent Company [Member]
Cash and cash equivalents	2.5	3.0	$ 3.1	$ 13.1
Receivables from subsidiaries	0.4	0.0
Notes receivable, net	46.4	40.0
Income taxes receivable	24.3	12.9
Investment in subsidiaries	286.0	237.7
Financial instruments owned, at fair value	3.0	0.0
Deferred tax assets, Net	12.0	16.6
Property and equipment, net	9.2	3.6
Goodwill and intangible assets, net	0.0	0.0
Other assets	13.1	4.9
Total assets	396.9	318.7
Adjustment to Investment in Subs for Equity Method Accounting	278.5	195.1
Liabilities:
Accounts payable and other accrued liabilities	29.3	8.8
Payables to Customers	30.7	25.8
Payables to lenders under loans	31.6	15.0
Payable to subsidiaries, net	123.7	61.7
Senior unsecured notes	45.5	45.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Total liabilities	260.8	156.8
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; 20,184,556 issued and 18,812,803 outstanding at September 30, 2015 and 19,826,635 issued and 18,883,662 outstanding at September 30, 2014	0.2	0.2
Common stock in treasury, at cost - 1,371,753 shares at September 30, 2015 and 942,973 shares at September 30, 2014	26.8	17.5
Additional Paid in Capital	240.8	229.6
Retained earnings	(78.1)	(50.4)
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	136.1	161.9
Total liabilities and stockholders' equity	$ 396.9	$ 318.7