Pledged Assets (Accepted Collateral) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Fair value of accepted collateral	$ 998.2	$ 707.8
Derivative contract
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Fair value of accepted collateral	34.6	16.8
DERIVATIVE CONTRACTS
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Fair value of accepted collateral	$ 0.0	$ 0.0