Number of Loans and Leases Modified in TDRs and Total Recorded Investments and Unpaid Principal Balances (Detail) - Class of Financing Receivable $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) Contract	Jun. 30, 2014 USD ($) Contract	Jun. 30, 2015 USD ($) Contract	Jun. 30, 2014 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases | Contract	21	38	80	74
Recorded Investment	$ 9.2	$ 5.9	$ 17.1	$ 10.1
Unpaid Principal Balance	$ 10.1	$ 7.0	$ 21.1	$ 11.4
Commercial
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases | Contract	1	2	3	3
Recorded Investment	$ 0.1	$ 0.7	$ 0.9	$ 1.4
Unpaid Principal Balance	$ 0.1	$ 1.0	$ 0.9	$ 1.7
Commercial | Commercial and Institutional
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases | Contract	1	2	2	2
Recorded Investment	$ 0.1	$ 0.7	$ 0.1	$ 0.7
Unpaid Principal Balance	$ 0.1	$ 1.0	$ 0.1	$ 1.0
Commercial | Commercial Real Estate
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases | Contract			1	1
Recorded Investment			$ 0.8	$ 0.7
Unpaid Principal Balance			$ 0.8	$ 0.7
Personal
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases | Contract	20	36	77	71
Recorded Investment	$ 9.1	$ 5.2	$ 16.2	$ 8.7
Unpaid Principal Balance	$ 10.0	$ 6.0	$ 20.2	$ 9.7
Personal | Residential Real Estate
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases | Contract	19	34	76	68
Recorded Investment	$ 8.5	$ 4.9	$ 15.6	$ 8.4
Unpaid Principal Balance	$ 9.4	$ 5.7	$ 19.6	$ 9.4
Personal | Private Client
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases | Contract	1	2	1	3
Recorded Investment	$ 0.6	$ 0.3	$ 0.6	$ 0.3
Unpaid Principal Balance	$ 0.6	$ 0.3	$ 0.6	$ 0.3