Acquisitions - Fillmore Advisors, LLC Acquisition (Details) - USD ($) $ in Millions		12 Months Ended
	Sep. 01, 2019	Sep. 30, 2020	Sep. 30, 2019
Business Acquisition [Line Items]
Acquired finite-lived intangible asset, weighted average useful life		1 year
Preliminary Purchase Price Allocation
Goodwill		$ 54.7	$ 51.2
Fillmore Advisors, LLC [Member]
Business Acquisition [Line Items]
Percentage of stock purchased	100.00%
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
Preliminary 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
Purchase price allocated to client base intangible assets acquired	0.7
Total excess purchase price allocated to identifiable intangible assets	0.7
Goodwill	1.9
Cash [Member] | Fillmore Advisors, LLC [Member]
Business Acquisition [Line Items]
Fair value of contingent consideration	$ 1.8