Revenue from Contracts with Clients (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Trust, investment and other servicing fees	$ 3,995.0	$ 3,852.1	$ 3,753.7
Security commissions and trading income	133.2	103.6	98.3
Other operating income	194.0	145.5	127.5
Securities Commission and Trading
Disaggregation of Revenue [Line Items]
Trust, investment and other servicing fees	102.4	87.1	86.7
Products and Services, Other Operating Income
Disaggregation of Revenue [Line Items]
Trust, investment and other servicing fees	$ 42.8	$ 41.8	$ 44.0