Intangible Assets - Intangible Assets Acquired During the Period (Details) $ in Millions	12 Months Ended
	Sep. 30, 2020 USD ($)
UOB Bullion and Futures Limited [Member] | Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 3.1
IFCM Commodities Acquisition [Member] | Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	1.0
Quest Capital Strategies Inc [Member] | Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	1.7
Gain Capital Holdings Inc [Member] | Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	9.8
Gain Capital Holdings Inc [Member] | Software Programs/Platforms [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	22.2
Gain Capital Holdings Inc [Member] | Trade/Domain Names [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	3.7
GIROXX [Member] | Software Programs/Platforms [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	1.5
GIROXX [Member] | Business Licenses [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 0.4