Acquisitions - Fillmore Advisors, LLC Acquisition (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2021	Sep. 30, 2020
Business Acquisition [Line Items]
Acquired finite-lived intangible asset, weighted average useful life		1 year
Purchase Price Allocation
Purchase price allocated to goodwill		$ 58.1	$ 54.7
Fillmore Advisors, LLC
Business Acquisition [Line Items]
Aggregate cash consideration	$ 1.4
Percentage of contingent earn-out payments	50.00%
Fair value of contingent consideration	$ 1.8
Acquired finite-lived intangible asset, weighted average useful life	5 years
Purchase Price Allocation
Cash and cash equivalents	$ 0.2
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties	0.3
Receivables from clients	0.2
Other assets	0.4
Total fair value of tangible assets acquired	1.1
Accounts payable and other accrued liabilities	0.5
Total fair value of tangible liabilities assumed	0.5
Fair value of identifiable net assets acquired	0.6
Purchase price	3.2
Total fair value of tangible assets acquired	2.6
Client relationships	0.7
Total excess purchase price allocated to identifiable intangible assets	0.7
Purchase price allocated to goodwill	1.9
Cash | Fillmore Advisors, LLC
Business Acquisition [Line Items]
Fair value of contingent consideration	$ 1.8