Impaired Loans by Segment and Class (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 303.1				$ 303.1				$ 269.8
Unpaid principal balance	357.6				357.6				325.4
Specific allowance	18.4				18.4				18.0
Average recorded investment	281.2	[1]	224.4	[1]	270.0	[1]	229.3	[1]
Interest income recognized	1.2		0.5		2.0		0.9
Commercial
Financing Receivable, Impaired [Line Items]
Recorded Investment	113.9				113.9				125.0
Unpaid principal balance	129.0				129.0				144.8
Specific allowance	14.5				14.5				11.0
Average recorded investment	101.4	[1]	93.4	[1]	99.5	[1]	100.4	[1]
Interest income recognized	0.4		0.2		0.6		0.3
Commercial | Commercial and Institutional
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	11.7				11.7				15.1
With a related specific reserve, recorded investment	14.2				14.2				8.1
With no related specific reserve, unpaid principal balance	14.9				14.9				19.3
With a related specific reserve, unpaid principal balance	16.3				16.3				10.2
Specific allowance	5.3				5.3				2.8
With no related specific reserve, average recorded investment	10.8	[1]	29.8	[1]	11.5	[1]	24.4	[1]
With a related specific reserve, average recorded investment	12.5	[1]	3.9	[1]	11.7	[1]	7.2	[1]
With no related specific reserve, interest income recognized	0.1		0.1		0.1		0.1
With a related specific reserve, interest income recognized
Commercial | Commercial Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	43.4				43.4				64.9
With a related specific reserve, recorded investment	40.0				40.0				32.3
With no related specific reserve, unpaid principal balance	50.8				50.8				76.9
With a related specific reserve, unpaid principal balance	42.4				42.4				33.8
Specific allowance	9.2				9.2				8.2
With no related specific reserve, average recorded investment	40.3	[1]	36.7	[1]	40.0	[1]	47.0	[1]
With a related specific reserve, average recorded investment	33.2	[1]	17.8	[1]	31.7	[1]	19.2	[1]
With no related specific reserve, interest income recognized	0.2		0.1		0.4		0.2
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				4.6
With no related specific reserve, unpaid principal balance	4.6				4.6				4.6
With no related specific reserve, average recorded investment	4.6	[1]	5.2	[1]	4.6	[1]	2.6	[1]
With no related specific reserve, interest income recognized	0.1				0.1
Personal
Financing Receivable, Impaired [Line Items]
Recorded Investment	189.2				189.2				144.8
Unpaid principal balance	228.6				228.6				180.6
Specific allowance	3.9				3.9				7.0
Average recorded investment	179.8	[1]	131.0	[1]	170.5	[1]	128.9	[1]
Interest income recognized	0.8		0.3		1.4		0.6
Personal | Residential Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	166.1				166.1				131.3
With a related specific reserve, recorded investment	8.2				8.2				11.8
With no related specific reserve, unpaid principal balance	204.7				204.7				165.7
With a related specific reserve, unpaid principal balance	8.8				8.8				13.0
Specific allowance	2.3				2.3				6.1
With no related specific reserve, average recorded investment	156.6	[1]	113.2	[1]	148.7	[1]	110.7	[1]
With a related specific reserve, average recorded investment	8.3	[1]	15.1	[1]	9.0	[1]	15.3	[1]
With no related specific reserve, interest income recognized	0.6		0.3		1.1		0.6
With a related specific reserve, interest income recognized
Personal | Private Client
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	13.3				13.3				0.8
With a related specific reserve, recorded investment	1.6				1.6				0.9
With no related specific reserve, unpaid principal balance	13.4				13.4				1.0
With a related specific reserve, unpaid principal balance	1.7				1.7				0.9
Specific allowance	1.6				1.6				0.9
With no related specific reserve, average recorded investment	13.3	[1]	1.8	[1]	11.4	[1]	1.6	[1]
With a related specific reserve, average recorded investment	1.6	[1]	0.9	[1]	1.4	[1]	1.3	[1]
With no related specific reserve, interest income recognized	0.2				0.3
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.