Securities and Commodity Financing Transactions - Netting of Securities Purchased Under Agreements (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Broker-Dealer [Abstract]
Securities purchased under agreements to resell, gross	$ 7,649.3	$ 4,609.9
Securities purchased under agreements to resell, offset	(4,669.8)	(2,937.9)
Securities purchased under agreements to resell, net	2,979.5	1,672.0
Securities borrowed, gross	1,129.1	1,209.8
Securities borrowed, net	1,129.1	1,209.8
Securities sold under agreements to repurchase, gross	9,196.4	6,133.5
Securities sold under agreements to repurchase, offset	(4,669.8)	(2,937.9)
Securities sold under agreements to repurchase, net	4,526.6	3,195.6
Securities loaned, gross	1,117.3	1,189.5
Securities loaned, net	$ 1,117.3	$ 1,189.5

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