Loans and Leases - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended										9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Lower Limit	Sep. 30, 2014 Upper Limit	Sep. 30, 2014 First Lien	Dec. 31, 2013 First Lien	Sep. 30, 2014 Equity	Dec. 31, 2013 Equity	Sep. 30, 2014 Nonperforming Financing Receivable	Dec. 31, 2013 Nonperforming Financing Receivable	Sep. 30, 2014 Performing Financing Receivable	Dec. 31, 2013 Performing Financing Receivable	Sep. 30, 2014 Performing Financing Receivable Lower Limit	Jun. 30, 2014 Residential Real Estate Re-Defaulted Loan	Jun. 30, 2013 Residential Real Estate Re-Defaulted Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral required on Residential real estate loan, lower limit	65.00%		65.00%
Collateral required on Residential real estate loan, upper limit	80.00%		80.00%
Draw period for credit line products			10 years
Equity credit lines										$ 1,800,000,000	$ 2,000,000,000
Loans held by Northern trust								88.00%	87.00%
Other U.S. loans and non-U.S. loans, short duration advances	1,400,000,000		1,400,000,000		1,300,000,000
Demand deposits	109,000,000		109,000,000		104,100,000
Loans held for sale	2,100,000		2,100,000		0
Days contractually past due for interest and principal and the loan is not well-secured and in the process of collection						90 days
Number of days past due for disclosure purposes to be categorized as a current							29 days
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	2,200,000	2,800,000	7,100,000	8,000,000
Undrawn loan commitments and standby letters of credit	3,500,000		3,500,000		3,400,000
Total Loans and Leases	$ 30,719,900,000		$ 30,719,900,000		$ 29,385,500,000							$ 58,600,000	$ 72,700,000	$ 96,900,000	$ 89,800,000
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
Number of Loans and Leases																	0	0