Acquisitions - GMP Securities, LLC (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
	Jan. 14, 2019	Sep. 30, 2020	Mar. 31, 2019	Sep. 30, 2019
Net assets acquired based on fair values:
Bargain purchase gain		$ 81.8	$ 5.4
GMP Securities Acquisition [Member]
Business Acquisition [Line Items]
Percentage of stock purchased	100.00%
Total merger consideration	$ 8.2
Business combination, excess amount above book value of tangible assets acquired	2.0
Operating revenues post-acquisition results of the acquired business				$ 8.2
Net income (loss) post-acquisition results of the acquired business				2.1
Net assets acquired based on fair values:
Cash and cash equivalents	1.1
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties	7.7
Financial instruments owned, at fair value	7.1
Deferred income taxes, net	2.7
Property and equipment	0.7
Other assets	0.7
Total fair value of tangible assets acquired	20.0
Accounts payable and other accrued liabilities	1.9
Payable to broker-dealers, clearing organizations, and counterparties	0.1
Financial instruments sold, not yet purchased, at fair value	4.4
Total fair value of tangible liabilities assumed	6.4
Fair value of identifiable net assets acquired	13.6
Purchase price	8.2
Bargain purchase gain	$ 5.4			$ 5.4