Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 3,767.2	$ 3,155.5
Securities loaned	1,747.2	1,441.9
Gross amount of secured financing	5,514.4	4,597.4
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	1,065.0	815.8
U.S. government agency obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	199.8	279.5
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	37.8	18.0
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	2,317.6	1,990.0
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	147.0	52.2
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 1,747.2	$ 1,441.9