Pledged Assets, Accepted Collateral and Restricted Assets (Accepted Collateral) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Financial Instruments Owned and Pledged as Collateral [Line Items]
Collateral that may be repledged or sold	$ 2.9
Total Collateral Accepted	65,394.8	$ 65,334.7
Reverse repurchase agreements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Collateral that may be repledged or sold	65,391.9	65,311.1
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	65,000.0	64,800.0
Derivative contracts
Financial Instruments Owned and Pledged as Collateral [Line Items]
Collateral that may be repledged or sold	$ 2.9	$ 23.6