Revenue from Contracts with Clients (Contract Receivables) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Total Client Receivables	$ 1,085.4	$ 935.8
Trust Fees Receivable, net
Disaggregation of Revenue [Line Items]
Total Client Receivables	960.6	819.3
Contract with customer, asset, allowance for credit loss	10.5	7.2
Other
Disaggregation of Revenue [Line Items]
Total Client Receivables	$ 124.8	$ 116.5