Securities and Commodity Financing Transactions - Netting of Securities Purchased Under Agreements (Details) - USD ($) $ in Millions	Mar. 31, 2023	Sep. 30, 2022
Broker-Dealer [Abstract]
Securities purchased under agreements to resell, gross	$ 7,932.2	$ 4,609.9
Securities purchased under agreements to resell, offset	(5,309.0)	(2,937.9)
Securities purchased under agreements to resell, net	2,623.2	1,672.0
Securities borrowed, gross	753.7	1,209.8
Securities borrowed, net	753.7	1,209.8
Securities sold under agreements to repurchase, gross	10,332.1	6,133.5
Securities sold under agreements to repurchase, offset	(5,309.0)	(2,937.9)
Securities sold under agreements to repurchase, net	5,023.1	3,195.6
Securities loaned, gross	764.5	1,189.5
Securities loaned, net	$ 764.5	$ 1,189.5

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Amount, after the effects of master netting arrangements, of securities borrowed from entities in exchange for collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

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