Impaired Loans by Segment and Class (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 245.2				$ 242.2
Unpaid principal balance	301.7				301.0
Specific allowance	8.4				6.6
Average recorded investment	246.9	[1]	294.1	[1]
Interest income recognized	0.6		1.0
Commercial
Financing Receivable, Impaired [Line Items]
Recorded Investment	85.3				78.9
Unpaid principal balance	96.0				93.5
Specific allowance	6.8				5.8
Average recorded investment	81.8	[1]	97.0	[1]
Interest income recognized	0.3		0.4
Commercial | Commercial and Institutional
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	7.9				9.0
With a related specific reserve, recorded investment	12.5				6.5
With no related specific reserve, unpaid principal balance	9.9				12.0
With a related specific reserve, unpaid principal balance	9.9				6.6
Specific allowance	4.0				2.9
With no related specific reserve, average recorded investment	9.2	[1]	11.4	[1]
With a related specific reserve, average recorded investment	8.5	[1]	10.0	[1]
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	49.1				47.0
With a related specific reserve, recorded investment	12.4				12.2
With no related specific reserve, unpaid principal balance	54.1				52.4
With a related specific reserve, unpaid principal balance	18.7				18.3
Specific allowance	2.8				2.9
With no related specific reserve, average recorded investment	48.5	[1]	44.8	[1]
With a related specific reserve, average recorded investment	12.2	[1]	26.4	[1]
With no related specific reserve, interest income recognized	0.2		0.3
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	3.4				4.2
With no related specific reserve, unpaid principal balance	3.4				4.2
With no related specific reserve, average recorded investment	3.4	[1]	4.4	[1]
With no related specific reserve, interest income recognized	0.1		0.1
Personal
Financing Receivable, Impaired [Line Items]
Recorded Investment	159.9				163.3
Unpaid principal balance	205.7				207.5
Specific allowance	1.6				0.8
Average recorded investment	165.1	[1]	197.1	[1]
Interest income recognized	0.3		0.6
Personal | Residential Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	153.1				160.9
With a related specific reserve, recorded investment	5.8				1.4
With no related specific reserve, unpaid principal balance	198.7				204.8
With a related specific reserve, unpaid principal balance	6.0				1.4
Specific allowance	1.2				0.4
With no related specific reserve, average recorded investment	161.3	[1]	189.2	[1]
With a related specific reserve, average recorded investment	2.8	[1]	7.1	[1]
With no related specific reserve, interest income recognized	0.3		0.6
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				0.2
With a related specific reserve, recorded investment	0.8				0.8
With no related specific reserve, unpaid principal balance	0.2				0.5
With a related specific reserve, unpaid principal balance	0.8				0.8
Specific allowance	0.4				0.4
With no related specific reserve, average recorded investment	0.2	[1]	0.7	[1]
With a related specific reserve, average recorded investment	0.8	[1]	0.1	[1]
With a related specific reserve, interest income recognized	$ 0		$ 0

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