Loans and Leases - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2015 Loan	Mar. 31, 2014 Loan	Dec. 31, 2014 USD ($)
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
Other U.S. loans and non-U.S. loans, short duration advances	$ 1,400,000,000		$ 1,400,000,000				$ 1,500,000,000
Demand deposits	46,700,000		46,700,000				92,100,000
Loans held for sale	3,900,000		3,900,000				2,500,000
Interest income that would have been recorded for nonperforming loans	2,000,000	$ 2,400,000	4,100,000	$ 4,900,000
Undrawn loan commitments and standby letters of credit	2,600,000		2,600,000				2,400,000
Total Loans and Leases	32,953,800,000		32,953,800,000				31,640,200,000
Unpaid principal balance	268,900,000		268,900,000				$ 301,000,000
Foreclosed residential real estate properties with carrying value	8,400,000		8,400,000
Consumer loans with carrying value collateralized by residential real estate property	24,400,000		$ 24,400,000
Interest Income
Accounts, Notes, Loans and Financing Receivable [Line Items]
Leveraged leases residual value impairment	17,800,000
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
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
Limit of individual homogeneous loans collective evaluation for impairment and excluded from impaired loan disclosures	$ 250,000		$ 250,000
First Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held by Northern trust	89.00%		89.00%				89.00%
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	$ 1,700,000,000		$ 1,700,000,000				$ 1,800,000,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	82,200,000		82,200,000				82,700,000
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	46,900,000		$ 46,900,000				$ 68,600,000
Performing Financing Receivable | Lower Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
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
Residential Real Estate | Re-Defaulted
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans and Leases | Loan					4	3
Recorded Investment	600,000	500,000	$ 600,000	500,000
Unpaid principal balance	$ 700,000	$ 900,000	$ 700,000	$ 900,000
Private Client | Re-Defaulted
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans and Leases | Loan						1