Loans and Leases - Borrower Ratings (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	$ 32,592.2	$ 33,822.1
1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	17,763.2	18,079.6
4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	14,419.6	15,255.1
6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	409.4	487.4
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	14,558.0	15,666.7
Commercial | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	8,067.1	8,965.5
Commercial | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	6,399.1	6,569.7
Commercial | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	91.8	131.5
Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	9,042.2	9,287.4
Commercial | Commercial and Institutional | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	5,832.9	6,187.2
Commercial | Commercial and Institutional | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	3,133.4	3,013.9
Commercial | Commercial and Institutional | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	75.9	86.3
Commercial | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	3,482.7	4,002.5
Commercial | Real Estate | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	1,280.7	1,825.7
Commercial | Real Estate | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	2,187.5	2,134.8
Commercial | Real Estate | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	14.5	42.0
Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	229.2	293.9
Commercial | Lease Financing, net | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	191.4	214.3
Commercial | Lease Financing, net | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	37.8	79.6
Commercial | Lease Financing, net | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	0.0	0.0
Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	1,538.5	1,877.8
Commercial | Non-U.S. | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	606.6	602.8
Commercial | Non-U.S. | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	930.5	1,273.5
Commercial | Non-U.S. | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	1.4	1.5
Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	265.4	205.1
Commercial | Other | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	155.5	135.5
Commercial | Other | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	109.9	67.9
Commercial | Other | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	0.0	1.7
Personal
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	18,034.2	18,155.4
Personal | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	9,696.1	9,114.1
Personal | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	8,020.5	8,685.4
Personal | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	317.6	355.9
Personal | Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	7,247.6	8,077.5
Personal | Real Estate | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	2,960.5	2,723.8
Personal | Real Estate | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	3,978.8	5,008.5
Personal | Real Estate | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	308.3	345.2
Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	10,753.1	10,052.0
Personal | Private Client | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	6,716.0	6,373.2
Personal | Private Client | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	4,027.8	3,668.4
Personal | Private Client | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	9.3	10.4
Personal | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	33.5	25.9
Personal | Other | 1 TO 3 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	19.6	17.1
Personal | Other | 4 TO 5 CATEGORY
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	13.9	8.5
Personal | Other | 6 TO 9 CATEGORY (WATCH LIST)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	$ 0.0	$ 0.3