Revenue from Contracts with Clients (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 961.5	$ 955.5	$ 1,965.1	$ 1,884.4
Security commissions and trading income	33.2	23.4	74.9	46.7
Other operating income	56.5	38.9	90.9	67.9
Securities Commissions and Trading Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	26.1	19.8	55.1	40.8
Other Operating Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 10.0	$ 10.5	$ 20.8	$ 20.4