Loans and Leases - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($) Loan	Dec. 31, 2015 USD ($) Loan	Dec. 31, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Lower limit of generally required loan to collateral value for residential real estate loans	65.00%
Upper limit of generally required loan to collateral value for residential real estate loans	80.00%
Draw period for credit line products	10 years
Other U.S. loans and non-U.S. loans, short duration advances	$ 1,400.0	$ 719.5
Demand deposits	88.1	75.4
Loans held for sale	13.4	12.0
Leases held for sale	43.0	112.3
Interest income that would have been recorded for nonperforming loans	8.5	8.1	$ 9.1
Undrawn loan commitments and standby letters of credit	2.3	3.1
Foreclosed residential real estate properties, carrying value	4.6	7.9
Consumer loans collateralized by residential properties in foreclosure	25.9	24.5
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	$ 1,200.0	$ 1,500.0
Loans held by Northern trust	91.00%	89.00%
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Modifications, Recorded Investment	$ 42.4	$ 37.9
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Modifications, Recorded Investment	$ 85.2	$ 79.2
Redefaulted \| Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans or leases modified in trouble debt restructuring \| Loan	5	10

Line of credit draw down period for equity credit line products.

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- Definition

Loans and Leases Receivable, Commitment to Lend

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- Definition

Loans and Leases Receivable, Non Performing Loans, Interest That Would Have Been Recognized

+ References

No definition available.

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- Definition

This element represents short duration advances, primarily related to overdrafts associated with the timing of custody client's investments included in other U.S. and non-U.S. loans.

+ References

No definition available.

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- Definition

Lower Limit of Generally Required Loan to Collateral Value for Residential Real Estate Loans

+ References

No definition available.

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- Definition

The amount of overdrawn money in accounts that may bear interest and that the depositor is entitled to withdraw at any time without prior notice, which have been reclassified to loans.

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No definition available.

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- Definition

Percentage of Total Equity Credit Lines

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No definition available.

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- Definition

Upper Limit of Generally Required Loan to Collateral Value for Residential Real Estate loans

+ References

No definition available.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

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- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

Amount before allowance of consumer real estate loans. Includes, but not limited to, construction financing, residential mortgage loans, and home equity lines of credit. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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- Definition

Amount, after allowance for bad debt, of mortgage loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

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Amount, after allowance for bad debt, of other loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

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- Definition

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

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