Pledged Assets, Accepted Collateral and Restricted Assets (Accepted Collateral) (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Financial Instruments Owned and Pledged as Collateral [Line Items]
Collateral that may be repledged or sold	$ 10.2
Total Collateral Accepted	98,550.2	$ 90,478.1
Reverse repurchase agreements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Collateral that may be repledged or sold	98,540.0	90,475.4
Collateral that may not be repledged or sold	0.0	0.0
Fair value of securities received as collateral that have been resold or repledged	96,800.0	89,700.0
Derivative contracts
Financial Instruments Owned and Pledged as Collateral [Line Items]
Collateral that may be repledged or sold	$ 10.2	$ 2.7