Securities and Commodity Financing Transactions - Netting of Securities Purchased Under Agreements (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020
Brokers and Dealers [Abstract]
Securities purchased under agreements to resell, gross	$ 2,027.2	$ 1,696.2
Securities purchased under agreements to resell, offset	0.0	0.0
Securities purchased under agreements to resell, net	2,027.2	1,696.2
Securities borrowed, gross	1,697.3	1,440.0
Securities borrowed, net	1,697.3	1,440.0
Securities sold under agreements to repurchase, gross	3,627.6	3,155.5
Securities sold under agreements to repurchase, offset	0.0	0.0
Securities sold under agreements to repurchase, net	3,627.6	3,155.5
Securities loaned, gross	1,700.3	1,441.9
Securities loaned, net	$ 1,700.3	$ 1,441.9

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

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