Capital and Other Regulatory Requirements - Regulatory Capital Requirements (Details) $ in Millions	Jun. 30, 2019 USD ($)
FCStone LLC [Member]
Net Capital under Commodity Exchange Act Computation	$ 174.4
Required Net Capital under Commodity Exchange Act	92.0
Cash and Securities Segregated under Commodity Exchange Act Regulation	2,407.4
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	2,349.8
Secured Funds	141.1
Secured Funds Required Under Commodity Exchange Act	125.0
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	4.7
INTL FCStone Ltd [Member]
Net Capital under Commodity Exchange Act Computation	236.3
Required Net Capital under Commodity Exchange Act	115.6
Cash and Securities Segregated under Commodity Exchange Act Regulation	210.9
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	206.3
INTL Netherlands BV [Member]
Net Capital under Commodity Exchange Act Computation	238.4
Required Net Capital under Commodity Exchange Act	116.6
INTL FCStone DTVM Ltda [Member]
Capital Required for Capital Adequacy	2.2
Capital	13.2
Gainvest S.A. Sociedad Gerente de FCI - Comision Nacional de Valores [Member]
Net Capital under Commodity Exchange Act Computation	0.5
Required Net Capital under Commodity Exchange Act	0.1
Capital Required for Capital Adequacy	0.1
Capital	4.5
INTL Capital S.A. – General Inspector of Justice [Member] [Domain]
Net Capital under Commodity Exchange Act Computation	0.7
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	0.5
Capital	5.4
INTL FCStone Financial (Canada) Inc. [Member] [Member]
Net Capital under Commodity Exchange Act Computation	1.8
Required Net Capital under Commodity Exchange Act	0.2
Cash and Securities Segregated under Commodity Exchange Act Regulation	0.0
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	0.0
Customer Reserve Requirement [Member] [Member]
Cash Reserve Deposit Required and Made	15.2
Cash Reserve Deposit Required	9.4
PAB Reserve Requirement [Member]
Cash Reserve Deposit Required and Made	0.0
Cash Reserve Deposit Required	$ 0.0