Revenue from Contracts with Clients (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 1,075.4	$ 961.5	$ 2,139.1	$ 1,965.1
Security commissions and trading income	33.0	33.2	67.8	74.9
Other operating income	54.4	56.5	109.3	90.9
Securities Commissions and Trading Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	27.2	26.1	55.9	55.1
Other Operating Income
Disaggregation of Revenue [Line Items]
Revenue from contract with client	$ 12.7	$ 10.0	$ 25.5	$ 20.8