Capital and Other Regulatory Requirements - Regulatory Capital Requirements (Details) $ in Millions	Mar. 31, 2019 USD ($)
FCStone LLC [Member]
Net Capital under Commodity Exchange Act Computation	$ 165.3
Required Net Capital under Commodity Exchange Act	85.5
Cash and Securities Segregated under Commodity Exchange Act Regulation	2,120.7
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	2,064.7
Secured Funds	129.7
Secured Funds Required Under Commodity Exchange Act	114.1
Sterne Agee, and Leach, Inc [Member]
Required Net Capital under Commodity Exchange Act	0.4
Sterne Agee Financial Services, Inc. [Member]
Net Capital under Commodity Exchange Act Computation	4.9
INTL FCStone Ltd [Member]
Net Capital under Commodity Exchange Act Computation	235.7
Required Net Capital under Commodity Exchange Act	111.8
Cash and Securities Segregated under Commodity Exchange Act Regulation	169.9
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	175.3
INTL Netherlands BV [Member]
Net Capital under Commodity Exchange Act Computation	239.5
Required Net Capital under Commodity Exchange Act	112.7
INTL FCStone DTVM Ltda [Member]
Capital Required for Capital Adequacy	0.4
Capital	12.4
INTL FCStone Banco de Cambio S.A. [Member] [Member]
Capital Required for Capital Adequacy	1.7
Capital	3.1
Gainvest S.A. Sociedad Gerente de FCI - Comision Nacional de Valores [Member]
Net Capital under Commodity Exchange Act Computation	0.7
Required Net Capital under Commodity Exchange Act	0.1
Capital Required for Capital Adequacy	0.1
Capital	4.2
INTL Capital S.A. – General Inspector of Justice [Member] [Domain]
Net Capital under Commodity Exchange Act Computation	1.2
Required Net Capital under Commodity Exchange Act	0.2
INTL Capital S.A. – Superintendence of Securities Markets of Buenos Aires [Member] [Domain]
Capital Required for Capital Adequacy	0.4
Capital	5.1
Customer Reserve Requirement [Member] [Member]
Cash Reserve Deposit Required and Made	0.0
Cash Reserve Deposit Required	0.0
PAB Reserve Requirement [Member]
Cash Reserve Deposit Required and Made	0.0
Cash Reserve Deposit Required	$ 0.0