Securities and Commodity Financing Transactions - Schedule of Netting of Securities Purchased Under Agreements (Details) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024
Broker-Dealer [Abstract]
Securities purchased under agreements to resell, gross	$ 13,623.0	$ 13,148.1
Securities purchased under agreements to resell, offset	(8,143.8)	(7,946.6)
Securities purchased under agreements to resell, net	5,479.2	5,201.5
Securities borrowed, gross	2,120.7	1,662.3
Securities borrowed, net	2,120.7	1,662.3
Securities sold under agreements to repurchase, gross	17,016.7	16,527.9
Securities sold under agreements to repurchase, offset	(8,143.8)	(7,946.6)
Securities sold under agreements to repurchase, net	8,872.9	8,581.3
Securities loaned, gross	1,826.5	1,615.9
Securities loaned, net	$ 1,826.5	$ 1,615.9

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