Impaired Loans by Segment and Class (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Financing Receivable, Impaired [Line Items]
Recorded investment	$ 273.0		$ 273.0		$ 279.4
Unpaid principal balance	343.2		343.2		356.9
Specific allowance	22.1		22.1		32.8
Average recorded investment	253.3	232.5	237.2	292.8
Interest income recognized	0.3	0.8	1.2	2.0
Commercial
Financing Receivable, Impaired [Line Items]
Recorded investment	124.7		124.7		121.2
Unpaid principal balance	160.5		160.5		162.6
Specific allowance	12.0		12.0		22.9
Average recorded investment	114.2	112.2	105.0	155.6
Interest income recognized	0.1	0.3	0.4	0.4
Commercial | Commercial and Institutional
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	26.3		26.3		21.4
With a related specific reserve, recorded investment	5.2		5.2		11.9
With no related specific reserve, unpaid principal balance	30.8		30.8		24.0
With a related specific reserve, unpaid principal balance	7.1		7.1		20.5
Specific allowance	3.2		3.2		8.8
With no related specific reserve, average recorded investment	27.0	20.1	25.2	21.0
With a related specific reserve, average recorded investment	5.2	12.7	6.5	29.1
With no related specific reserve, interest income recognized	0.1		0.2
With a related specific reserve, interest income recognized
Commercial | Commercial Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	59.1		59.1		46.5
With a related specific reserve, recorded investment	28.9		28.9		41.4
With no related specific reserve, unpaid principal balance	84.2		84.2		68.0
With a related specific reserve, unpaid principal balance	33.2		33.2		50.1
Specific allowance	8.8		8.8		14.1
With no related specific reserve, average recorded investment	54.7	34.2	49.6	34.5
With a related specific reserve, average recorded investment	22.1	45.2	20.2	71.0
With no related specific reserve, interest income recognized		0.3	0.2	0.4
With a related specific reserve, interest income recognized
Commercial | Lease Financing, net
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	5.2		5.2
With no related specific reserve, unpaid principal balance	5.2		5.2
With no related specific reserve, average recorded investment	5.2		3.5
Personal
Financing Receivable, Impaired [Line Items]
Recorded investment	148.3		148.3		158.2
Unpaid principal balance	182.7		182.7		194.3
Specific allowance	10.1		10.1		9.9
Average recorded investment	139.1	120.3	132.2	137.2
Interest income recognized	0.2	0.5	0.8	1.6
Personal | Residential Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	129.4		129.4		134.4
With a related specific reserve, recorded investment	15.8		15.8		18.9
With no related specific reserve, unpaid principal balance	162.6		162.6		162.6
With a related specific reserve, unpaid principal balance	17.0		17.0		26.2
Specific allowance	9.2		9.2		8.9
With no related specific reserve, average recorded investment	120.7	107.9	114.0	123.2
With a related specific reserve, average recorded investment	15.3	7.5	15.3	8.3
With no related specific reserve, interest income recognized	0.2	0.5	0.8	1.6
With a related specific reserve, interest income recognized
Personal | Private Client
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	2.2		2.2		1.6
With a related specific reserve, recorded investment	0.9		0.9		3.3
With no related specific reserve, unpaid principal balance	2.2		2.2		1.9
With a related specific reserve, unpaid principal balance	0.9		0.9		3.6
Specific allowance	0.9		0.9		1.0
With no related specific reserve, average recorded investment	2.2	3.2	1.8	3.5
With a related specific reserve, average recorded investment	0.9	1.7	1.1	2.2
With a related specific reserve, interest income recognized