Impaired Loans by Segment and Class (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	$ 247.6		$ 279.4
Unpaid Principal Balance	318.9		356.9
Specific Allowance	22.0		32.8
Average Recorded Investment	234.1	285.4
Interest Income Recognized	0.4	0.9
Commercial
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	109.9		121.2
Unpaid Principal Balance	143.6		162.6
Specific Allowance	12.9		22.9
Average Recorded Investment	107.4	160.2
Interest Income Recognized	0.1	0.1
Commercial | Commercial and Institutional | Impaired Financing Receivables with Related Allowance
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	9.6		11.9
Unpaid Principal Balance	12.9		20.5
Specific Allowance	8.6		8.8
Average Recorded Investment	10.5	35.2
Commercial | Commercial and Institutional | Impaired Financing Receivables with No Related Allowance
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	20.9		21.4
Unpaid Principal Balance	23.9		24.0
Average Recorded Investment	18.9	17.0
Commercial | Commercial Real Estate | Impaired Financing Receivables with Related Allowance
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	20.4		41.4
Unpaid Principal Balance	27.0		50.1
Specific Allowance	4.3		14.1
Average Recorded Investment	20.7	82.6
Commercial | Commercial Real Estate | Impaired Financing Receivables with No Related Allowance
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	59.0		46.5
Unpaid Principal Balance	79.8		68.0
Average Recorded Investment	57.3	25.4
Interest Income Recognized	0.1	0.1
Personal
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	137.7		158.2
Unpaid Principal Balance	175.3		194.3
Specific Allowance	9.1		9.9
Average Recorded Investment	126.7	125.2
Interest Income Recognized	0.3	0.8
Personal | Residential Real Estate | Impaired Financing Receivables with Related Allowance
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	15.5		18.9
Unpaid Principal Balance	16.6		26.2
Specific Allowance	8.1		8.9
Average Recorded Investment	15.6	6.7
Personal | Residential Real Estate | Impaired Financing Receivables with No Related Allowance
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	119.0		134.4
Unpaid Principal Balance	154.8		162.6
Average Recorded Investment	108.1	113.3
Interest Income Recognized	0.3	0.8
Personal | Private Client | Impaired Financing Receivables with Related Allowance
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	1.7		3.3
Unpaid Principal Balance	2.0		3.6
Specific Allowance	1.0		1.0
Average Recorded Investment	1.7	3.1
Personal | Private Client | Impaired Financing Receivables with No Related Allowance
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	1.5		1.6
Unpaid Principal Balance	1.9		1.9
Average Recorded Investment	$ 1.3	$ 2.1