Loans and Leases - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Y	Jun. 30, 2012	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Draw period for credit line products			10
Other U.S. loans and non-U.S. loans, short duration advances	$ 1,600,000,000		$ 1,600,000,000		$ 1,500,000,000
Demand deposits	69,500,000		69,500,000		224,700,000
Loans held for sale	1,100,000		1,100,000		11,700,000
Maximum limit of individual homogeneous loans collective evaluation for impairment and excluded from impaired loan disclosures	250,000		250,000
Interest income that would have been recorded for nonperforming loans	2,600,000	2,700,000	5,200,000	5,800,000
Unfunded loan commitments and standby letters of credit	4,100,000		4,100,000		2,100,000
Total Loans and Leases	28,810,400,000		28,810,400,000		29,504,500,000
Lower Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days contractually past due for interest and principal and the loan is not well-secured and in the process of collection			90 days
Period for which troubled debt restructurings with modified terms is performing before ceasing to be classified as impaired loan	6 months
Troubled debt restructuring loans and leases receivable to Performing status	6 months
Upper Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of days past due for disclosure purposes to be categorized as a current			29 days
First Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held by Northern trust	86.00%		86.00%		86.00%
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	2,100,000,000		2,100,000,000		2,300,000,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	51,900,000		51,900,000		49,800,000
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	94,900,000		94,900,000		74,700,000
Re-Defaulted
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number loans modified in troubled debt restructurings in the previous 12 months which became nonperforming	0		0
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral required on Residential real estate loan	65.00%		65.00%
Collateral required on Residential real estate loan	80.00%		80.00%
Residential Real Estate | Re-Defaulted
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number loans modified in troubled debt restructurings in the previous 12 months which became nonperforming		2		2
Recorded Investment				100,000
Unpaid Principal Balance				$ 100,000