Revenue from Contracts with Clients (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Trust, investment and other servicing fees	$ 4,361.8	$ 4,432.6	$ 4,361.1
Security commissions and trading income	135.0	136.2	140.2
Other operating income	228.7	191.3	243.9
Securities Commission and Trading
Disaggregation of Revenue [Line Items]
Trust, investment and other servicing fees	115.9	116.8	112.1
Products and Services, Other Operating Income
Disaggregation of Revenue [Line Items]
Trust, investment and other servicing fees	$ 38.9	$ 37.2	$ 53.9