Impaired Loans by Segment and Class (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 294.3				$ 269.8
Unpaid principal balance	350.3				325.4
Specific allowance	20.3				18.0
Average recorded investment	259.1	[1]	234.1	[1]
Interest income recognized	0.8		0.4
Commercial
Financing Receivable, Impaired [Line Items]
Recorded Investment	103.9				125.0
Unpaid principal balance	121.8				144.8
Specific allowance	13.5				11.0
Average recorded investment	97.8	[1]	107.4	[1]
Interest income recognized	0.2		0.1
Commercial | Commercial and Institutional
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	13.8				15.1
With a related specific reserve, recorded investment	11.0				8.1
With no related specific reserve, unpaid principal balance	17.9				19.3
With a related specific reserve, unpaid principal balance	13.1				10.2
Specific allowance	4.7				2.8
With no related specific reserve, average recorded investment	12.2	[1]	18.9	[1]
With a related specific reserve, average recorded investment	11.0	[1]	10.5	[1]
With a related specific reserve, interest income recognized
Commercial | Commercial Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	44.0				64.9
With a related specific reserve, recorded investment	30.5				32.3
With no related specific reserve, unpaid principal balance	53.6				76.9
With a related specific reserve, unpaid principal balance	32.6				33.8
Specific allowance	8.8				8.2
With no related specific reserve, average recorded investment	39.6	[1]	57.3	[1]
With a related specific reserve, average recorded investment	30.4	[1]	20.7	[1]
With no related specific reserve, interest income recognized	0.2		0.1
With a related specific reserve, interest income recognized
Commercial | Lease Financing, net
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	4.6				4.6
With no related specific reserve, unpaid principal balance	4.6				4.6
With no related specific reserve, average recorded investment	4.6	[1]
Personal
Financing Receivable, Impaired [Line Items]
Recorded Investment	190.4				144.8
Unpaid principal balance	228.5				180.6
Specific allowance	6.8				7.0
Average recorded investment	161.3	[1]	126.7	[1]
Interest income recognized	0.6		0.3
Personal | Residential Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	165.3				131.3
With a related specific reserve, recorded investment	9.8				11.8
With no related specific reserve, unpaid principal balance	201.9				165.7
With a related specific reserve, unpaid principal balance	11.1				13.0
Specific allowance	5.2				6.1
With no related specific reserve, average recorded investment	141.0	[1]	108.1	[1]
With a related specific reserve, average recorded investment	9.8	[1]	15.6	[1]
With no related specific reserve, interest income recognized	0.5		0.3
With a related specific reserve, interest income recognized
Personal | Private Client
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	13.7				0.8
With a related specific reserve, recorded investment	1.6				0.9
With no related specific reserve, unpaid principal balance	13.8				1.0
With a related specific reserve, unpaid principal balance	1.7				0.9
Specific allowance	1.6				0.9
With no related specific reserve, average recorded investment	9.4	[1]	1.3	[1]
With a related specific reserve, average recorded investment	1.1	[1]	1.7	[1]
With no related specific reserve, interest income recognized	0.1
With a related specific reserve, interest income recognized

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