Acquisitions - UOB Bullion and Futures Limited (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2019	Sep. 30, 2021	Sep. 30, 2020
Business Acquisition [Line Items]
Purchase price allocated to goodwill		$ 58.1	$ 54.7
Acquired finite-lived intangible asset, weighted average useful life		1 year
UOB Bullion and Futures Limited
Business Acquisition [Line Items]
Purchase price	$ 5.0
Payable to clients	351.8
Purchase price allocated to property and equipment acquired	0.8
Purchase price allocated to client base intangible assets acquired	3.1
Purchase price allocated to goodwill	$ 1.1
Operating revenues post-acquisition results of the acquired business			10.3
Net income (loss) post-acquisition results of the acquired business			$ (1.4)
UOB Bullion and Futures Limited | Client Base Intangible Assets
Business Acquisition [Line Items]
Acquired finite-lived intangible asset, weighted average useful life	5 years