Securities and Commodity Financing Transactions - Collateral Maturities of Gross Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2022	Sep. 30, 2022
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 10,302.3	$ 6,133.5
Securities loaned	483.9	1,189.5
Gross amount of secured financing	10,786.2	7,323.0
Overnight and Open
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	2,856.6	3,664.7
Securities loaned	483.9	1,189.5
Gross amount of secured financing	3,340.5	4,854.2
Less than 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	7,358.8	2,279.1
Securities loaned	0.0	0.0
Gross amount of secured financing	7,358.8	2,279.1
30-90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	47.8	186.3
Securities loaned	0.0	0.0
Gross amount of secured financing	47.8	186.3
Over 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	39.1	3.4
Securities loaned	0.0	0.0
Gross amount of secured financing	$ 39.1	$ 3.4