Securities and Commodity Financing Transactions - Netting of Securities Purchased Under Agreements (Details) - USD ($) $ in Millions	Dec. 31, 2023	Sep. 30, 2023
Broker-Dealer [Abstract]
Securities purchased under agreements to resell, gross	$ 7,836.3	$ 7,649.3
Securities purchased under agreements to resell, offset	(4,036.5)	(4,669.8)
Securities purchased under agreements to resell, net	3,799.8	2,979.5
Securities borrowed, gross	994.5	1,129.1
Securities borrowed, net	994.5	1,129.1
Securities sold under agreements to repurchase, gross	10,090.7	9,196.4
Securities sold under agreements to repurchase, offset	(4,036.5)	(4,669.8)
Securities sold under agreements to repurchase, net	6,054.2	4,526.6
Securities loaned, gross	942.7	1,117.3
Securities loaned, net	$ 942.7	$ 1,117.3