Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2021	Sep. 30, 2021
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 4,091.7	$ 4,340.9
Securities loaned	2,747.1	2,153.6
Gross amount of secured financing	6,838.8	6,494.5
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	10.5	106.8
U.S. government agency obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	530.9	354.6
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	283.2	255.9
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	3,118.7	3,536.2
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	148.4	87.4
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 2,747.1	$ 2,153.6