Regulatory Requirements and Subsidiary Dividend Restrictions - Regulatory Capital Requirements (Details) - USD ($)	Sep. 30, 2017	Sep. 30, 2016
Minimum Net Capital Required for Entity	$ 0.08
Cash Held at Banks and Money Market Funds - Segregated	464,300,000	$ 515,200,000
INTL FCStone Financial [Member]
Net Capital	157,100,000
Cash and Securities Segregated under Commodity Exchange Act Regulation	2,248,000,000
Secured funds	165,100,000
Minimum Net Capital Required for Entity	74,000,000
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	2,195,700,000
Secured Funds Required Under Commodity Exchange Act	148,700,000
Cash Reserve Deposit Required and Made	7,200,000	0
Cash Held at Banks and Money Market Funds - Segregated	13,500,000	$ 200,000
Sterne Agee Clearing, Inc. [Member]
Net Capital	1,500,000
Minimum Net Capital Required for Entity	100,000
Sterne Agee Financial Services, Inc. [Member]
Net Capital	4,900,000
Minimum Net Capital Required for Entity	300,000
INTL FCStone, Ltd [Member]
Cash and Securities Segregated under Commodity Exchange Act Regulation	107,100,000
Net Capital under Commodity Exchange Act Computation	158,700,000
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	106,600,000
Required Net Capital under Commodity Exchange Act	88,300,000
INTL FCStone Europe [Member]
Net Capital under Commodity Exchange Act Computation	158,000,000
Required Net Capital under Commodity Exchange Act	88,300,000
INTL FCStone DTVM Ltda [Member]
Net Capital	13,000,000
Capital Required for Capital Adequacy	500,000
Gainvest S.A. Sociedad Gerente de FCI - Comision Nacional de Valores [Member]
Net Capital	5,100,000
Net Capital under Commodity Exchange Act Computation	400,000
Required Net Capital under Commodity Exchange Act	100,000
Capital Required for Capital Adequacy	200,000
INTL CIBSA S.A. - Comision Nacional de Valores [Member]
Net Capital	7,600,000
Net Capital under Commodity Exchange Act Computation	0
Required Net Capital under Commodity Exchange Act	0
Capital Required for Capital Adequacy	$ 900,000