Impaired Loans by Segment and Class (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 270.4				$ 270.4				$ 293.4
Unpaid principal balance	329.9				329.9				360.5
Specific allowance	9.5				9.5				10.4
Average recorded investment	265.5	[1]	294.7	[1]	279.3	[1]	278.3	[1]
Interest income recognized	1.2		0.8		3.1		2.8
Commercial
Financing Receivable, Impaired [Line Items]
Recorded Investment	97.9				97.9				99.5
Unpaid principal balance	112.6				112.6				123.2
Specific allowance	8.6				8.6				8.1
Average recorded investment	91.0	[1]	102.5	[1]	93.3	[1]	100.6	[1]
Interest income recognized	0.4		0.3		1.0		0.9
Commercial | Commercial and Institutional
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	11.6				11.6				12.2
With a related specific reserve, recorded investment	20.5				20.5				9.6
With no related specific reserve, unpaid principal balance	14.6				14.6				18.1
With a related specific reserve, unpaid principal balance	20.5				20.5				12.1
Specific allowance	5.1				5.1				3.6
With no related specific reserve, average recorded investment	12.6	[1]	10.7	[1]	11.5	[1]	11.3	[1]
With a related specific reserve, average recorded investment	11.9	[1]	14.1	[1]	10.6	[1]	12.5	[1]
With no related specific reserve, interest income recognized	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	49.2				49.2				46.6
With a related specific reserve, recorded investment	12.4				12.4				26.7
With no related specific reserve, unpaid principal balance	54.9				54.9				57.1
With a related specific reserve, unpaid principal balance	18.4				18.4				31.5
Specific allowance	3.5				3.5				4.5
With no related specific reserve, average recorded investment	48.5	[1]	42.0	[1]	46.0	[1]	40.6	[1]
With a related specific reserve, average recorded investment	13.7	[1]	31.2	[1]	20.9	[1]	31.6	[1]
With no related specific reserve, interest income recognized	0.2		0.2		0.7		0.6
With a related specific reserve, interest income recognized
Commercial | Lease Financing, net
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	4.2				4.2				4.4
With no related specific reserve, unpaid principal balance	4.2				4.2				4.4
With no related specific reserve, average recorded investment	4.3	[1]	4.5	[1]	4.3	[1]	4.6	[1]
With no related specific reserve, interest income recognized	0.1		0.1		0.2		0.2
Personal
Financing Receivable, Impaired [Line Items]
Recorded Investment	172.5				172.5				193.9
Unpaid principal balance	217.3				217.3				237.3
Specific allowance	0.9				0.9				2.3
Average recorded investment	174.5	[1]	192.2	[1]	186.0	[1]	177.7	[1]
Interest income recognized	0.8		0.5		2.1		1.9
Personal | Residential Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	170.2				170.2				185.0
With a related specific reserve, recorded investment	1.3				1.3				8.1
With no related specific reserve, unpaid principal balance	214.5				214.5				227.8
With a related specific reserve, unpaid principal balance	1.5				1.5				8.7
Specific allowance	0.4				0.4				2.3
With no related specific reserve, average recorded investment	172.0	[1]	168.1	[1]	180.8	[1]	155.2	[1]
With a related specific reserve, average recorded investment	1.3	[1]	10.0	[1]	4.1	[1]	9.4	[1]
With no related specific reserve, interest income recognized	0.8		0.3		2.1		1.4
With a related specific reserve, interest income recognized
Personal | Private Client
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	0.1				0.1				0.8
With a related specific reserve, recorded investment	0.9				0.9
With no related specific reserve, unpaid principal balance	0.4				0.4				0.8
With a related specific reserve, unpaid principal balance	0.9				0.9
Specific allowance	0.5				0.5
With no related specific reserve, average recorded investment	0.3	[1]	13.3	[1]	0.6	[1]	12.0	[1]
With a related specific reserve, average recorded investment	0.9	[1]	0.8	[1]	0.5	[1]	1.1	[1]
With no related specific reserve, interest income recognized			0.2				0.5
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.