Pledged and Restricted Assets - Accepted Collateral (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Fair value of accepted collateral	$ 707.8	$ 426.2
Repurchase agreements
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	605.0
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	$ 16.8	$ 15.4