Revenue from Contracts with Clients (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 1,003.8	$ 975.5	$ 2,968.9	$ 2,859.9
Security commissions and trading income	26.0	29.1	100.9	75.8
Other operating income	53.5	45.1	144.4	113.0
Securities Commissions and Trading Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	21.8	23.5	76.9	64.3
Other Operating Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 10.8	$ 10.5	$ 31.6	$ 30.9