Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Loans and Leases
Evaluated on an Individual Level	$ 103.9	$ 92.2
Evaluated on a Collective Level	37,733.8	31,317.4
Total Loans and Leases	37,837.7	31,409.6
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Level	5.7	5.0
Evaluated on a Collective Level	141.5	99.5
Allowance Assigned to Loans and Leases	147.2	104.5	$ 114.5
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Level	3.0	1.9
Evaluated on a Collective Level	37.4	18.0
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	40.4	19.9	24.9
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	187.6	124.4
COMMERCIAL
Loans and Leases
Evaluated on an Individual Level	35.6	10.4
Evaluated on a Collective Level	19,701.2	13,990.9
Total Loans and Leases	19,736.8	14,001.3
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Level	5.3	3.4
Evaluated on a Collective Level	89.4	54.7
Allowance Assigned to Loans and Leases	94.7	58.1	58.1
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Level	3.0	1.9
Evaluated on a Collective Level	35.8	13.9
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	38.8	15.8	20.5
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	133.5	73.9
PERSONAL
Loans and Leases
Evaluated on an Individual Level	68.3	81.8
Evaluated on a Collective Level	18,032.6	17,326.5
Total Loans and Leases	18,100.9	17,408.3
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Level	0.4	1.6
Evaluated on a Collective Level	52.1	44.8
Allowance Assigned to Loans and Leases	52.5	46.4	56.4
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Level	0.0	0.0
Evaluated on a Collective Level	1.6	4.1
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	1.6	4.1	$ 4.4
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	$ 54.1	$ 50.5