Securities and Commodity Financing Transactions - Schedule of Netting of Securities Purchased Under Agreements (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024
Broker-Dealer [Abstract]
Securities purchased under agreements to resell, gross	$ 19,374.5	$ 13,148.1
Securities purchased under agreements to resell, offset	(11,215.2)	(7,946.6)
Securities purchased under agreements to resell, net	8,159.3	5,201.5
Securities borrowed, gross	2,486.2	1,662.3
Securities borrowed, offset	0.0	0.0
Securities borrowed, net	2,486.2	1,662.3
Securities sold under agreements to repurchase, gross	24,590.6	16,527.9
Securities sold under agreements to repurchase, offset	(11,215.2)	(7,946.6)
Securities sold under agreements to repurchase, net	13,375.4	8,581.3
Securities loaned, gross	2,032.1	1,615.9
Securities loaned, offset	0.0	0.0
Securities loaned, net	$ 2,032.1	$ 1,615.9

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