Condensed Parent Only Financial Statements - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Cash and cash equivalents	$ 316.2	$ 268.1	$ 231.3	$ 156.1
Deposits and receivables from broker-dealers, clearing organizations and counterparties	237.0	277.6
Income taxes receivable	1.1	10.6
Financial instruments owned, at fair value	1,606.1	1,421.9
Deferred tax assets, Net	34.5	28.2
Property and equipment, net	29.4	19.7
Total assets	5,951.3	5,070.0	3,039.7
Liabilities:
Accounts payable and other accrued liabilities	161.3	144.8
Payables to Customers	2,854.2	2,593.5
Payables to lenders under loans	182.8	41.6
Senior unsecured notes	45.5	45.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	839.4	568.3
Total liabilities	5,517.5	4,672.9
INTL FCStone Inc. (Parent Company Only) stockholders’ equity:
Common stock, $0.01 par value. Authorized 30,000,000 shares; 20,557,175 issued and 18,435,218 outstanding at September 30, 2016 and 20,184,556 issued and 18,812,803 outstanding at September 30, 2015	0.2	0.2
Common stock in treasury, at cost - 2,121,957 shares at September 30, 2016 and 1,371,753 shares at September 30, 2015	(46.3)	(26.8)
Additional Paid in Capital	249.4	240.8
Retained earnings	255.1	200.4
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	433.8	397.1
Total liabilities and stockholders' equity	5,951.3	5,070.0
Parent Company [Member]
Cash and cash equivalents	1.3	2.5	$ 3.0	$ 3.1
Deposits and receivables from broker-dealers, clearing organizations and counterparties	2.9	0.0
Receivables from subsidiaries	3.6	0.4
Notes receivable, net	6.9	46.4
Income taxes receivable	14.0	24.3
Investment in subsidiaries	316.3	286.0
Financial instruments owned, at fair value	1.3	3.0
Deferred tax assets, Net	15.7	12.0
Property and equipment, net	12.7	9.2
Other assets	17.2	13.1
Total assets	391.9	396.9
Liabilities:
Accounts payable and other accrued liabilities	27.7	29.3
Payables to Customers	4.6	30.7
Payables to lenders under loans	139.3	31.6
Payable to subsidiaries, net	17.1	123.7
Senior unsecured notes	45.5	45.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	35.9	0.0
Total liabilities	270.1	260.8
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; 20,557,175 issued and 18,435,218 outstanding at September 30, 2016 and 20,184,556 issued and 18,812,803 outstanding at September 30, 2015	0.2	0.2
Common stock in treasury, at cost - 2,121,957 shares at September 30, 2016 and 1,371,753 shares at September 30, 2015	(46.3)	(26.8)
Additional Paid in Capital	249.4	240.8
Retained earnings	(81.5)	(78.1)
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	121.8	136.1
Total liabilities and stockholders' equity	391.9	396.9
Adjustment to Investment in Subs for Equity Method Accounting	$ 336.6	$ 278.5