Securities and Commodity Financing Transactions - Netting of Securities Purchased Under Agreements (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023
Broker-Dealer [Abstract]
Securities purchased under agreements to resell, gross	$ 9,807.9	$ 7,649.3
Securities purchased under agreements to resell, offset	(6,063.3)	(4,669.8)
Securities purchased under agreements to resell, net	3,744.6	2,979.5
Securities borrowed, gross	1,430.6	1,129.1
Securities borrowed, net	1,430.6	1,129.1
Securities sold under agreements to repurchase, gross	12,075.0	9,196.4
Securities sold under agreements to repurchase, offset	(6,063.3)	(4,669.8)
Securities sold under agreements to repurchase, net	6,011.7	4,526.6
Securities loaned, gross	1,458.6	1,117.3
Securities loaned, net	$ 1,458.6	$ 1,117.3

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