Securities and Commodity Financing Transactions - Schedule of Netting of Securities Purchased Under Agreements (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Broker-Dealer [Abstract]
Securities purchased under agreements to resell, gross	$ 26,298.1	$ 13,148.1
Securities purchased under agreements to resell, offset	(15,972.7)	(7,946.6)
Securities purchased under agreements to resell, net	10,325.4	5,201.5
Securities borrowed, gross	2,743.1	1,662.3
Securities borrowed, net	2,743.1	1,662.3
Securities sold under agreements to repurchase, gross	29,523.7	16,527.9
Securities sold under agreements to repurchase, offset	(15,972.7)	(7,946.6)
Securities sold under agreements to repurchase, net	13,551.0	8,581.3
Securities loaned, gross	2,550.8	1,615.9
Securities loaned, net	$ 2,550.8	$ 1,615.9

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