Regulatory Requirements and Subsidiary Dividend Restrictions - Regulatory Capital Requirements (Details) - USD ($)	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Minimum Net Capital Required for Entity		$ 0.08
Cash Held at Banks and Money Market Funds - Segregated	$ 765,400,000	$ 464,300,000
INTL FCStone Financial [Member]
Net Capital	167,700,000
Cash and Securities Segregated under Commodity Exchange Act Regulation	2,637,000,000
Secured funds	148,900,000
Minimum Net Capital Required for Entity	101,400,000
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	2,579,700,000
Secured Funds Required Under Commodity Exchange Act	132,800,000
Cash Reserve Deposit Required and Made			$ 6,400,000
Cash Held at Banks and Money Market Funds - Segregated	300,000
Sterne Agee Clearing, Inc. [Member]
Net Capital	0
Minimum Net Capital Required for Entity	0
Sterne Agee Financial Services, Inc. [Member]
Net Capital	4,600,000
Minimum Net Capital Required for Entity	400,000
INTL FCStone, Ltd [Member]
Cash and Securities Segregated under Commodity Exchange Act Regulation	187,300,000
Net Capital under Commodity Exchange Act Computation	193,100,000
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	182,100,000
Required Net Capital under Commodity Exchange Act	95,900,000
INTL FCStone Europe [Member]
Net Capital under Commodity Exchange Act Computation	193,000,000
Required Net Capital under Commodity Exchange Act	95,900,000
INTL FCStone DTVM Ltda [Member]
Net Capital	11,200,000
Capital Required for Capital Adequacy	2,100,000
Gainvest S.A. Sociedad Gerente de FCI - Comision Nacional de Valores [Member]
Net Capital	4,200,000
Net Capital under Commodity Exchange Act Computation	1,200,000
Required Net Capital under Commodity Exchange Act	100,000
Capital Required for Capital Adequacy	100,000
INTL CIBSA S.A. - Comision Nacional de Valores [Member]
Net Capital	4,400,000
Net Capital under Commodity Exchange Act Computation	0
Required Net Capital under Commodity Exchange Act	0
Capital Required for Capital Adequacy	$ 500,000