Intangible Assets - Intangible Assets Acquired During the Period (Details) - USD ($) $ in Millions		12 Months Ended
	Aug. 03, 2021	Sep. 30, 2021	Sep. 30, 2020
Business licenses | Global Payments
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets acquired		$ 0.5
Chasing Returns Limited | Software programs/platforms
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 2.4	2.4
UOB Bullion and Futures Limited | Customer base
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired			$ 3.1
IFCM Commodities Acquisition | Customer base
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired			1.0
Quest Capital Strategies Inc | Customer base
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired			1.7
Gain Capital Holdings Inc | Software programs/platforms
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired		22.2
Gain Capital Holdings Inc | Customer base
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired			9.8
Gain Capital Holdings Inc | Trade/domain names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired			3.7
GIROXX | Software programs/platforms
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired		$ 1.5
GIROXX | Business licenses
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets acquired			$ 0.4