Securities and Commodity Financing Transactions - Collateral Maturities of Gross Obligations (Details) - USD ($) $ in Millions	Jun. 30, 2022	Sep. 30, 2021
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 5,601.7	$ 4,340.9
Securities loaned	1,095.5	2,153.6
Gross amount of secured financing	6,697.2	6,494.5
Overnight and Open
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	4,510.1	2,949.8
Securities loaned	1,095.5	2,153.6
Gross amount of secured financing	5,605.6	5,103.4
Less than 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	773.9	973.4
Securities loaned	0.0	0.0
Gross amount of secured financing	773.9	973.4
30-90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	238.2	137.5
Securities loaned	0.0	0.0
Gross amount of secured financing	238.2	137.5
Over 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	79.5	280.2
Securities loaned	0.0	0.0
Gross amount of secured financing	$ 79.5	$ 280.2