Capital and Other Regulatory Requirements - Regulatory Capital Requirements (Details) - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018
FCStone LLC [Member]
Net Capital under Commodity Exchange Act Computation	$ 157.0
Required Net Capital under Commodity Exchange Act		$ 93.3
Cash and Securities Segregated under Commodity Exchange Act Regulation	2,452.9
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated		2,397.0
Secured Funds	162.6
Secured Funds Required Under Commodity Exchange Act		144.8
Sterne Agee Clearing, Inc. [Member]
Net Capital under Commodity Exchange Act Computation	2.0
Required Net Capital under Commodity Exchange Act		0.1
Sterne Agee, and Leach, Inc [Member]
Net Capital under Commodity Exchange Act Computation	3.7
Required Net Capital under Commodity Exchange Act		0.5
INTL FCStone Ltd [Member]
Net Capital under Commodity Exchange Act Computation	193.2
Required Net Capital under Commodity Exchange Act		91.7
Cash and Securities Segregated under Commodity Exchange Act Regulation	167.4
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated		163.4
INTL Netherlands BV [Member]
Net Capital under Commodity Exchange Act Computation	192.4
Required Net Capital under Commodity Exchange Act		91.7
INTL FCStone DTVM Ltda [Member]
Net Capital	11.6
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	1.4
Required Net Capital under Commodity Exchange Act		0.1
Net Capital	4.7
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		0.6
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		0.4
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