Number of Loans and Leases Modified in TDRs and Total Recorded Investments and Unpaid Principal Balances (Detail) (Class of Financing Receivable, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Contract	Dec. 31, 2013 Contract
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	138	203
Recorded Investment	$ 19.8	$ 93.1
Unpaid Principal Balance	24.0	113.8
Commercial
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	10	26
Recorded Investment	4.6	31.1
Unpaid Principal Balance	5.6	40.9
Commercial | Commercial and Institutional
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	2	14
Recorded Investment	0.7	3.4
Unpaid Principal Balance	0.8	4.7
Commercial | Commercial Real Estate Portfolio Segment
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	8	12
Recorded Investment	3.9	27.7
Unpaid Principal Balance	4.8	36.2
Personal
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	128	177
Recorded Investment	15.2	62.0
Unpaid Principal Balance	18.4	72.9
Personal | Residential Real Estate
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	124	168
Recorded Investment	15.0	49.1
Unpaid Principal Balance	17.9	60.0
Personal | Private Client
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	4	9
Recorded Investment	0.2	12.9
Unpaid Principal Balance	$ 0.5	$ 12.9