Condensed Parent Only Financial Statements - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Cash and cash equivalents	$ 314.9	$ 316.2	$ 268.1	$ 231.3
Deposits and receivables from broker-dealers, clearing organizations and counterparties	2,625.1	1,761.4
Income taxes receivable	0.4	1.1
Financial instruments owned, at fair value	1,731.8	1,606.1
Deferred tax assets, Net	42.6	34.5
Property and equipment, net	38.7	29.4
Total assets	6,243.4	5,950.3	5,070.0
Liabilities:
Accounts payable and other accrued liabilities	135.6	161.3
Payables to Customers	3,072.9	2,854.2
Payables to lenders under loans	230.2	182.8
Senior unsecured notes	0.0	44.5	45.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	717.6	839.4
Total liabilities	5,793.5	5,516.5
INTL FCStone Inc. (Parent Company Only) stockholders’ equity:
Common stock, $0.01 par value. Authorized 30,000,000 shares; 20,855,243 issued and 18,733,286 outstanding at September 30, 2017 and 20,557,175 issued and 18,435,218 outstanding at September 30, 2016	0.2	0.2
Common stock in treasury, at cost - 2,121,957 shares at September 30, 2017 and 2016	(46.3)	(46.3)
Additional Paid in Capital	259.0	249.4
Retained earnings	261.5	255.1
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	449.9	433.8
Total liabilities and stockholders' equity	6,243.4	5,950.3
Parent Company [Member]
Cash and cash equivalents	2.0	1.3	$ 2.5	$ 3.0
Deposits and receivables from broker-dealers, clearing organizations and counterparties	0.0	2.9
Receivables from subsidiaries	3.8	3.6
Notes receivable, net	4.8	6.9
Income taxes receivable	8.6	14.0
Investment in subsidiaries	312.3	316.3
Financial instruments owned, at fair value	0.0	1.3
Deferred tax assets, Net	26.5	15.7
Property and equipment, net	24.8	12.7
Other assets	7.6	16.2
Total assets	390.4	390.9
Liabilities:
Accounts payable and other accrued liabilities	19.8	27.7
Payables to Customers	2.1	4.6
Payables to lenders under loans	152.0	139.3
Payable to subsidiaries, net	49.4	17.1
Senior unsecured notes	0.0	44.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	25.3	35.9
Total liabilities	248.6	269.1
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; 20,855,243 issued and 18,733,286 outstanding at September 30, 2017 and 20,557,175 issued and 18,435,218 outstanding at September 30, 2016	0.2	0.2
Common stock in treasury, at cost - 2,121,957 shares at September 30, 2017 and 2016	(46.3)	(46.3)
Additional Paid in Capital	259.0	249.4
Retained earnings	(71.1)	(81.5)
Total INTL FCStone Inc. (Parent Company Only) stockholders’ equity	141.8	121.8
Total liabilities and stockholders' equity	390.4	390.9
Adjustment to Investment in Subs for Equity Method Accounting	$ 332.6	$ 336.6