Loans and Leases - Additional Information (Detail) (USD $)	9 Months Ended					3 Months Ended	9 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2012 Y	Dec. 31, 2011	Sep. 30, 2012 Residential Real Estate	Sep. 30, 2012 Nonperforming Financing Receivable	Dec. 31, 2011 Nonperforming Financing Receivable	Sep. 30, 2012 Nonperforming Financing Receivable Residential Real Estate Contract	Sep. 30, 2012 Nonperforming Financing Receivable Residential Real Estate Contract	Sep. 30, 2012 Performing Financing Receivable	Dec. 31, 2011 Performing Financing Receivable	Sep. 30, 2012 Nonperforming Financing Receivable	Sep. 30, 2011 Nonperforming Financing Receivable Contract	Sep. 30, 2012 Nonperforming Financing Receivable	Sep. 30, 2011 Nonperforming Financing Receivable Contract	Sep. 30, 2012 Equity	Dec. 31, 2011 Equity	Sep. 30, 2012 First Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral required on Residential real estate loan			65.00%
Collateral required on Residential real estate loan			80.00%
Draw period for credit line products	10
Equity credit lines														$ 2,400,000,000	$ 2,600,000,000
Loans held by Northern trust																86.00%
Other U.S. loans and non-U.S. loans, short duration advances	1,800,000,000	1,600,000,000
Demand deposits	46,900,000	191,600,000
Loans held for sale	0	9,300,000
Maximum limit of individual homogeneous loans collective evaluation for impairment and excluded from impaired loan disclosures	250,000
Interest income recorded for nonperforming loans										2,900,000	3,700,000	8,700,000	11,800,000
Unfunded loan commitments and standby letters of credit	3,500,000	9,700,000
Total Loans and Leases	29,542,700,000	29,063,900,000		45,700,000	72,200,000			63,500,000	41,100,000
Number of loans modified as TDRs in the previous 12 months which became nonperforming						0	2				2		8
Recorded Investment						89,400	89,400				300,000		2,200,000
Unpaid Principal Balance						$ 89,400	$ 89,400				$ 700,000		$ 3,100,000