Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 4,340.9	$ 3,155.5
Securities loaned	2,153.6	1,441.9
Gross amount of secured financing	6,494.5	4,597.4
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	106.8	815.8
U.S. government agency obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	354.6	279.5
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	255.9	18.0
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	3,536.2	1,990.0
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	87.4	52.2
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 2,153.6	$ 1,441.9