Pledged Assets, Accepted Collateral and Restricted Assets (Accepted Collateral) (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Total Collateral Accepted	$ 90,478.1	$ 65,334.7
Fair value of securities received as collateral that have been resold or repledged	89,700.0	64,800.0
Reverse repurchase agreements
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Collateral that may be repledged or sold	90,475.4	65,311.1
Reverse repurchase agreements	0.0	0.0
Derivative contracts
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Collateral that may be repledged or sold	$ 2.7	$ 23.6