Capital and Other Regulatory Requirements - Regulatory Capital Requirements (Details) $ in Millions	Dec. 31, 2016 USD ($)
FCStone LLC [Member]
Net Capital under Commodity Exchange Act Computation	$ 129.3
Required Net Capital under Commodity Exchange Act	80.2
Cash and Securities Segregated under Commodity Exchange Act Regulation	2,130.4
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	2,079.0
Secured Funds	126.5
Secured Funds Required Under Commodity Exchange Act	109.8
Sterne Agee Clearing, Inc. [Member]
Net Capital under Commodity Exchange Act Computation	1.1
Required Net Capital under Commodity Exchange Act	0.1
Sterne Agee, and Leach, Inc [Member]
Net Capital under Commodity Exchange Act Computation	26.7
Required Net Capital under Commodity Exchange Act	2.0
Sterne Agee Financial Services, Inc. [Member]
Net Capital under Commodity Exchange Act Computation	4.7
Required Net Capital under Commodity Exchange Act	0.4
INTL FCStone Ltd [Member]
Net Capital under Commodity Exchange Act Computation	160.1
Required Net Capital under Commodity Exchange Act	85.5
Cash and Securities Segregated under Commodity Exchange Act Regulation	58.9
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated	52.4
INTL Netherlands BV [Member]
Net Capital under Commodity Exchange Act Computation	155.5
Required Net Capital under Commodity Exchange Act	84.4
INTL FCStone DTVM Ltda [Member]
Net Capital	2.5
Capital Required for Capital Adequacy	0.5
Gainvest S.A. Sociedad Gerente de FCI - Comision Nacional de Valores [Member]
Net Capital under Commodity Exchange Act Computation	4.0
Required Net Capital under Commodity Exchange Act	0.1
Net Capital	8.0
Capital Required for Capital Adequacy	0.1
INTL Capital S.A – Rosario Futures Exchanges [Member] [Domain]
Net Capital	14.8
Capital Required for Capital Adequacy	12.7
INTL Capital S.A. – General Inspector of Justice [Member] [Domain]
Net Capital under Commodity Exchange Act Computation	6.9
Required Net Capital under Commodity Exchange Act	1.0
INTL Capital S.A. – Superintendence of Securities Markets of Buenos Aires [Member] [Domain]
Net Capital under Commodity Exchange Act Computation	10.7
Required Net Capital under Commodity Exchange Act	$ 0.6