Capital and Other Regulatory Requirements - Regulatory Capital Requirements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
FCStone LLC [Member]
Net Capital under Commodity Exchange Act Computation	$ 158.0
Required Net Capital under Commodity Exchange Act		$ 89.4
Cash and Securities Segregated under Commodity Exchange Act Regulation	2,211.0
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated		2,151.6
Secured Funds	133.6
Secured Funds Required Under Commodity Exchange Act		118.0
Sterne Agee, and Leach, Inc [Member]
Net Capital under Commodity Exchange Act Computation	5.1
Required Net Capital under Commodity Exchange Act		0.4
INTL FCStone Ltd [Member]
Net Capital under Commodity Exchange Act Computation	186.5
Required Net Capital under Commodity Exchange Act		183.4
Cash and Securities Segregated under Commodity Exchange Act Regulation	237.5
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated		97.6
INTL Netherlands BV [Member]
Net Capital under Commodity Exchange Act Computation	236.5
Required Net Capital under Commodity Exchange Act		98.5
INTL FCStone DTVM Ltda [Member]
Net Capital	12.1
Capital Required for Capital Adequacy		2.2
Gainvest S.A. Sociedad Gerente de FCI - Comision Nacional de Valores [Member]
Net Capital under Commodity Exchange Act Computation	1.8
Required Net Capital under Commodity Exchange Act		0.1
Net Capital	4.1
Capital Required for Capital Adequacy		0.1
INTL Capital S.A. – General Inspector of Justice [Member] [Domain]
Net Capital under Commodity Exchange Act Computation	4.8
Required Net Capital under Commodity Exchange Act		0.5
INTL Capital S.A. – Superintendence of Securities Markets of Buenos Aires [Member] [Domain]
Net Capital under Commodity Exchange Act Computation	$ 0.9
Required Net Capital under Commodity Exchange Act		$ 0.3