Acquisitions - Coininvest and European Precious Metal Trading Acquisitions (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Apr. 30, 2019	Sep. 30, 2021	Sep. 30, 2019	Sep. 30, 2020
Business Acquisition [Line Items]
Acquired finite-lived intangible asset, weighted average useful life		1 year
Net assets acquired based on fair values:
Goodwill		$ 58.1		$ 54.7
Coininvest and EPMT Acquisitions
Business Acquisition [Line Items]
Total merger consideration	$ 22.0
Purchase of shareholders equity interest	$ 11.2
Acquired finite-lived intangible asset, weighted average useful life	5 years
Operating revenues post-acquisition results of the acquired business			$ 0.6
Net income (loss) post-acquisition results of the acquired business			$ (0.3)
Net assets acquired based on fair values:
Cash and cash equivalents	$ 2.0
Receivables from clients	1.2
Receivable from affiliate	1.1
Income tax receivable	0.1
Physical commodities inventory	9.8
Deferred tax assets, net	0.2
Other assets	1.2
Total fair value of tangible assets acquired	15.6
Accounts payable and other accrued liabilities	0.2
Payable to clients	0.2
Total fair value of tangible liabilities assumed	0.4
Fair value of identifiable net assets acquired	15.2
Purchase price	22.0
Total fair value of tangible assets acquired	6.8
Domain names	2.1
Identifiable intangible assets acquired	2.5
Total excess purchase price allocated to identifiable intangible assets	4.6
Goodwill	$ 2.2