Revenue from Contracts with Clients (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 1,063.7	$ 1,003.6
Security commissions and trading income	34.8	41.7
Other operating income	54.9	34.4
Securities Commissions and Trading Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	28.7	29.0
Other Operating Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 12.8	$ 10.8