Impaired Loans by Segment and Class (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 242.2		$ 293.4
Unpaid principal balance	301.0		360.5
Specific allowance	6.6		10.4
Average recorded investment	271.2	[1]	282.9	[1]
Interest income recognized	3.9		3.8
Commercial
Financing Receivable, Impaired [Line Items]
Recorded Investment	78.9		99.5
Unpaid principal balance	93.5		123.2
Specific allowance	5.8		8.1
Average recorded investment	90.1	[1]	102.8	[1]
Interest income recognized	1.3		1.3
Commercial | Commercial and Institutional
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	9.0		12.2
With a related specific reserve, recorded investment	6.5		9.6
With no related specific reserve, unpaid principal balance	12.0		18.1
With a related specific reserve, unpaid principal balance	6.6		12.1
Specific allowance	2.9		3.6
With no related specific reserve, average recorded investment	11.3	[1]	11.7	[1]
With a related specific reserve, average recorded investment	9.6	[1]	12.5	[1]
With no related specific reserve, interest income recognized	0.1		0.2
With a related specific reserve, interest income recognized	0		0
Commercial | Commercial Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	47.0		46.6
With a related specific reserve, recorded investment	12.2		26.7
With no related specific reserve, unpaid principal balance	52.4		57.1
With a related specific reserve, unpaid principal balance	18.3		31.5
Specific allowance	2.9		4.5
With no related specific reserve, average recorded investment	46.1	[1]	42.4	[1]
With a related specific reserve, average recorded investment	18.8	[1]	31.0	[1]
With no related specific reserve, interest income recognized	1.0		0.9
With a related specific reserve, interest income recognized	0		0
Commercial | Lease Financing, net
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	4.2		4.4
With no related specific reserve, unpaid principal balance	4.2		4.4
With no related specific reserve, average recorded investment	4.3	[1]	4.5	[1]
With a related specific reserve, average recorded investment			0.7	[1]
With no related specific reserve, interest income recognized	0.2		0.2
With a related specific reserve, interest income recognized	0		0
Personal
Financing Receivable, Impaired [Line Items]
Recorded Investment	163.3		193.9
Unpaid principal balance	207.5		237.3
Specific allowance	0.8		2.3
Average recorded investment	181.1	[1]	180.1	[1]
Interest income recognized	2.6		2.5
Personal | Residential Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	160.9		185.0
With a related specific reserve, recorded investment	1.4		8.1
With no related specific reserve, unpaid principal balance	204.8		227.8
With a related specific reserve, unpaid principal balance	1.4		8.7
Specific allowance	0.4		2.3
With no related specific reserve, average recorded investment	176.7	[1]	160.2	[1]
With a related specific reserve, average recorded investment	3.3	[1]	5.7	[1]
With no related specific reserve, interest income recognized	2.6		2.5
With a related specific reserve, interest income recognized	0		0
Personal | Private Client
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	0.2		0.8
With a related specific reserve, recorded investment	0.8
With no related specific reserve, unpaid principal balance	0.5		0.8
With a related specific reserve, unpaid principal balance	0.8
Specific allowance	0.4
With no related specific reserve, average recorded investment	0.5	[1]	10.2	[1]
With a related specific reserve, average recorded investment	0.6	[1]	4.0	[1]
With a related specific reserve, interest income recognized	$ 0		$ 0

[1] Average recorded investments in impaired loans are calculated as the average of the month-end impaired loan balances for the period.