Acquisitions - Quest Capital and Carl Kliem S.A. (Details) - USD ($) $ in Millions	1 Months Ended			12 Months Ended
	May 31, 2020	Aug. 31, 2019	Nov. 30, 2018	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Business Acquisition [Line Items]
Acquired finite-lived intangible asset, weighted average useful life				1 year
Bargain purchase gain				$ 3.3	$ 81.9	$ 5.5
Quest Capital Strategies Inc
Business Acquisition [Line Items]
Purchase price		$ 1.7
Quest Capital Strategies Inc | Customer Lists And Relationships
Business Acquisition [Line Items]
Acquired finite-lived intangible asset, weighted average useful life		7 years
Carl Kliem S.A.
Business Acquisition [Line Items]
Purchase price			$ 2.1
Cash and cash equivalents			1.7
Receivables from clients			1.1
Property and equipment, net			0.1
Income tax receivable			0.1
Accounts payable and other accrued liabilities			0.6
Payable to clients			$ 0.2
Bargain purchase gain						0.1
Operating revenues post-acquisition results of the acquired business						4.2
Net income (loss) post-acquisition results of the acquired business						$ (2.3)
GIROXX
Business Acquisition [Line Items]
Purchase price	$ 4.4
Cash and cash equivalents	6.5
Property and equipment, net	0.1
Accounts payable and other accrued liabilities	0.6
Payable to clients	$ 5.8
Operating revenues post-acquisition results of the acquired business					0.5
Net income (loss) post-acquisition results of the acquired business					$ (0.6)
GIROXX | Capitalized software development
Business Acquisition [Line Items]
Acquired finite-lived intangible asset, weighted average useful life	3 years