Revenue from Contracts with Clients (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 1,111.0	$ 1,003.8	$ 3,250.1	$ 2,968.9
Security commissions and trading income	36.5	26.0	104.3	100.9
Other operating income	62.3	53.5	171.6	144.4
Securities Commissions and Trading Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	26.1	21.8	82.0	76.9
Other Operating Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 14.0	$ 10.8	$ 39.5	$ 31.6