Contingent Liabilities	6 Months Ended
Jun. 30, 2016
Commitments and Contingencies Disclosure [Abstract]
Contingent Liabilities	Contingent Liabilities Commitments, Letters of Credit and Indemnifications. Northern Trust, in the normal course of business, enters into various types of commitments and issues letters of credit to meet the liquidity and credit enhancement needs of its clients. Legally binding commitments to extend credit generally have fixed expiration dates or other termination clauses. Since a significant portion of the commitments are expected to expire without being drawn upon, the total commitment amount does not necessarily represent future loans or liquidity requirements. Legally binding commitments to extend credit totaled $36.4 billion and $37.2 billion as of June 30, 2016 and December 31, 2015, respectively. Standby letters of credit obligate Northern Trust to meet certain financial obligations of its clients, if, under the contractual terms of the agreement, the clients are unable to do so. These instruments are primarily issued to support public and private financial commitments, including commercial paper, bond financing, initial margin requirements on futures exchanges and similar transactions. Northern Trust is obligated to meet the entire financial obligation of these agreements and in certain cases is able to recover the amounts paid through recourse against collateral received or other participants. Standby letters of credit outstanding were $4.1 billion as of June 30, 2016 and December 31, 2015. As part of its securities custody activities and at the direction of its clients, Northern Trust lends securities owned by clients to borrowers who are reviewed and approved by the Northern Trust Counterparty Risk Management Committee. In connection with these activities, Northern Trust has issued indemnifications to certain clients against certain losses that are a direct result of a borrower’s failure to return securities when due, should the value of such securities exceed the value of the collateral required to be posted. Borrowers are required to collateralize fully securities received with cash or marketable securities. As securities are loaned, collateral is maintained at a minimum of 100% of the fair value of the securities plus accrued interest. The collateral is revalued on a daily basis. The amount of securities loaned as of June 30, 2016 and December 31, 2015 subject to indemnification was $97.8 billion and $94.5 billion, respectively. Because of the credit quality of the borrowers and the requirement to collateralize fully securities borrowed, management believes that the exposure to credit loss from this activity is not significant and no liability was recorded as of June 30, 2016 or December 31, 2015, related to these indemnifications. Legal Proceedings. In the normal course of business, the Corporation and its subsidiaries are routinely defendants in or parties to a number of pending and threatened legal actions, including, but not limited to, actions brought on behalf of various claimants or classes of claimants, regulatory matters, employment matters and challenges from tax authorities regarding the amount of taxes due. In certain of these actions and proceedings, claims for substantial monetary damages or adjustments to recorded tax liabilities are asserted. Based on current knowledge, after consultation with legal counsel and after taking into account current accruals, management does not believe that losses, if any, arising from pending litigation or threatened legal actions or regulatory matters will have a material adverse effect on the consolidated financial position or liquidity of the Corporation, although such matters could have a material adverse effect on the Corporation’s operating results for a particular period. Under GAAP, (i) an event is “probable” if the “future event or events are likely to occur”; (ii) an event is “reasonably possible” if “the chance of the future event or events occurring is more than remote but less than likely”; and (iii) an event is “remote” if “the chance of the future event or events occurring is slight.” For the reasons set out in this paragraph, the outcome of some matters is inherently difficult to predict and/or the range of loss cannot be reasonably estimated. This may be the case in matters that (i) will be decided by a jury, (ii) are in early stages, (iii) involve uncertainty as to the likelihood of a class being certified or the ultimate size of the class, (iv) are subject to appeals or motions, (v) involve significant factual issues to be resolved, including with respect to the amount of damages, (vi) do not specify the amount of damages sought or (vii) seek very large damages based on novel and complex damage and liability legal theories. Accordingly, the Corporation cannot reasonably estimate the eventual outcome of these pending matters, the timing of their ultimate resolution or what the eventual loss, fines or penalties, if any, related to each pending matter will be. In accordance with applicable accounting guidance, the Corporation records accruals for litigation and regulatory matters when those matters present loss contingencies that are both probable and reasonably estimable. When loss contingencies are not both probable and reasonably estimable, the Corporation does not record accruals. No material accruals have been recorded for pending litigation or threatened legal actions or regulatory matters. For a limited number of the matters for which a loss is reasonably possible in future periods, whether in excess of an accrued liability or where there is no accrued liability, the Corporation is able to estimate a range of possible loss. As of June 30, 2016, the Corporation has estimated the upper end of the range of reasonably possible losses for these matters to be approximately $90 million in the aggregate. This aggregate amount of reasonably possible loss is based upon currently available information and is subject to significant judgment and a variety of assumptions and known and unknown uncertainties. The matters underlying the estimated range will change from time to time, and actual results will vary significantly from the current estimate. In certain other pending matters, there may be a range of reasonably possible losses (including reasonably possible losses in excess of amounts accrued) that cannot be reasonably estimated for the reasons described above. Such matters are not included in the estimate of reasonably possible losses identified above. A number of participants in Northern Trust’s securities lending program, which is associated with its asset servicing business, have commenced either individual lawsuits or purported class actions in which they claim, among other things, that Northern Trust failed to exercise prudence in the investment management of the collateral received from the borrowers of the securities, resulting in losses associated with the 2008 financial crisis that they seek to recover. The cases assert various contractual, statutory and common law claims, including claims for breach of fiduciary duty under common law and under the Employee Retirement Income Security Act (ERISA). In 2013, Northern Trust recorded a $19.2 million charge in connection with a settlement to resolve certain claims related to two of these lawsuits. In the second quarter of 2016, Northern Trust recorded a $46.5 million charge in connection with a settlement to resolve substantially all of the remaining claims. This settlement is not final as it requires further documentation, signed agreements and court approval. One individual lawsuit related to this matter is not subject to the settlement and remains pending. As previously disclosed in April 2014, Northern Trust remains subject to an investigation by the U.S. Securities and Exchange Commission (SEC) related to Northern Trust’s securities lending activities. Northern Trust continues to cooperate with the SEC in this investigation. In January 2015, the Public Prosecutor’s Office of France recommended that certain charges be brought against Northern Trust Fiduciary Services (Guernsey) Limited (NTFS), an indirect subsidiary of the Corporation, relating to the administration of two trusts for which NTFS serves as trustee. In April 2015, a French investigating magistrate judge charged NTFS with complicity in estate tax fraud. Charges also were brought against a number of other persons and entities related to this matter. As trustee, NTFS provided no tax advice and had no involvement in the preparation or filing of the challenged estate tax filings. NTFS has contested the criminal charge in the French court. After being postponed by the trial court in January 2016, the trial currently is scheduled for September 2016. Visa Class B Common Shares. Northern Trust, as a member of Visa U.S.A. Inc. (Visa U.S.A.) and in connection with the 2007 restructuring of Visa U.S.A. and its affiliates and the 2008 initial public offering of Visa Inc. (Visa), received certain Visa Class B common shares. The Visa Class B common shares are subject to certain selling restrictions until the final resolution of the covered litigation noted below, at which time the shares are convertible into Visa Class A common shares based on a conversion rate dependent upon the ultimate cost of resolving the covered litigation. Certain members of Visa U.S.A. are obligated to indemnify Visa for losses resulting from certain litigation relating to interchange fees (the covered litigation). On October 19, 2012, Visa signed a settlement agreement with plaintiff representatives for binding settlement of the covered litigation. On January 14, 2014, the United States District Court for the Eastern District of New York entered a final judgment order approving the settlement with the class plaintiffs. A number of objectors appealed from that order and more than 30 opt-out cases have been filed by merchants in various federal district courts. On June 30, 2016, the United States Court of Appeals for the Second Circuit reversed the District Court’s approval of the settlement and remanded the case to the District Court for further proceedings. The ultimate resolution of the covered litigation and the timing for removal of the selling restrictions on the Visa Class B common shares are uncertain. Northern Trust recorded a $123.1 million and $99.9 million net gain on the sale of 1.1 million and 1.0 million of its Visa Class B common shares for the three months ended June 30, 2016 and 2015, respectively. Northern Trust continued to hold approximately 4.1 million and 5.2 million Visa Class B common shares, which are recorded at their original cost basis of zero as of June 30, 2016 and 2015, respectively.

Contingent Liabilities

Commitments, Letters of Credit and Indemnifications. Northern Trust, in the normal course of business, enters into various types of commitments and issues letters of credit to meet the liquidity and credit enhancement needs of its clients.

Legally binding commitments to extend credit generally have fixed expiration dates or other termination clauses. Since a significant portion of the commitments are expected to expire without being drawn upon, the total commitment amount does not necessarily represent future loans or liquidity requirements. Legally binding commitments to extend credit totaled $36.4 billion and $37.2 billion as of June 30, 2016 and December 31, 2015, respectively.

Standby letters of credit obligate Northern Trust to meet certain financial obligations of its clients, if, under the contractual terms of the agreement, the clients are unable to do so. These instruments are primarily issued to support public and private financial commitments, including commercial paper, bond financing, initial margin requirements on futures exchanges and similar transactions. Northern Trust is obligated to meet the entire financial obligation of these agreements and in certain cases is able to recover the amounts paid through recourse against collateral received or other participants. Standby letters of credit outstanding were $4.1 billion as of June 30, 2016 and December 31, 2015.

As part of its securities custody activities and at the direction of its clients, Northern Trust lends securities owned by clients to borrowers who are reviewed and approved by the Northern Trust Counterparty Risk Management Committee. In connection with these activities, Northern Trust has issued indemnifications to certain clients against certain losses that are a direct result of a borrower’s failure to return securities when due, should the value of such securities exceed the value of the collateral required to be posted. Borrowers are required to collateralize fully securities received with cash or marketable securities. As securities are loaned, collateral is maintained at a minimum of 100% of the fair value of the securities plus accrued interest. The collateral is revalued on a daily basis. The amount of securities loaned as of June 30, 2016 and December 31, 2015 subject to indemnification was $97.8 billion and $94.5 billion, respectively. Because of the credit quality of the borrowers and the requirement to collateralize fully securities borrowed, management believes that the exposure to credit loss from this activity is not significant and no liability was recorded as of June 30, 2016 or December 31, 2015, related to these indemnifications.

Legal Proceedings. In the normal course of business, the Corporation and its subsidiaries are routinely defendants in or parties to a number of pending and threatened legal actions, including, but not limited to, actions brought on behalf of various claimants or classes of claimants, regulatory matters, employment matters and challenges from tax authorities regarding the amount of taxes due. In certain of these actions and proceedings, claims for substantial monetary damages or adjustments to recorded tax liabilities are asserted.

Based on current knowledge, after consultation with legal counsel and after taking into account current accruals, management does not believe that losses, if any, arising from pending litigation or threatened legal actions or regulatory matters will have a material adverse effect on the consolidated financial position or liquidity of the Corporation, although such matters could have a material adverse effect on the Corporation’s operating results for a particular period.

Under GAAP, (i) an event is “probable” if the “future event or events are likely to occur”; (ii) an event is “reasonably possible” if “the chance of the future event or events occurring is more than remote but less than likely”; and (iii) an event is “remote” if “the chance of the future event or events occurring is slight.”

For the reasons set out in this paragraph, the outcome of some matters is inherently difficult to predict and/or the range of loss cannot be reasonably estimated. This may be the case in matters that (i) will be decided by a jury, (ii) are in early stages, (iii) involve uncertainty as to the likelihood of a class being certified or the ultimate size of the class, (iv) are subject to appeals or motions, (v) involve significant factual issues to be resolved, including with respect to the amount of damages, (vi) do not specify the amount of damages sought or (vii) seek very large damages based on novel and complex damage and liability legal theories. Accordingly, the Corporation cannot reasonably estimate the eventual outcome of these pending matters, the timing of their ultimate resolution or what the eventual loss, fines or penalties, if any, related to each pending matter will be.

In accordance with applicable accounting guidance, the Corporation records accruals for litigation and regulatory matters when those matters present loss contingencies that are both probable and reasonably estimable. When loss contingencies are not both probable and reasonably estimable, the Corporation does not record accruals. No material accruals have been recorded for pending litigation or threatened legal actions or regulatory matters.

For a limited number of the matters for which a loss is reasonably possible in future periods, whether in excess of an accrued liability or where there is no accrued liability, the Corporation is able to estimate a range of possible loss. As of June 30, 2016, the Corporation has estimated the upper end of the range of reasonably possible losses for these matters to be approximately $90 million in the aggregate. This aggregate amount of reasonably possible loss is based upon currently available information and is subject to significant judgment and a variety of assumptions and known and unknown uncertainties. The matters underlying the estimated range will change from time to time, and actual results will vary significantly from the current estimate.

In certain other pending matters, there may be a range of reasonably possible losses (including reasonably possible losses in excess of amounts accrued) that cannot be reasonably estimated for the reasons described above. Such matters are not included in the estimate of reasonably possible losses identified above.

A number of participants in Northern Trust’s securities lending program, which is associated with its asset servicing business, have commenced either individual lawsuits or purported class actions in which they claim, among other things, that Northern Trust failed to exercise prudence in the investment management of the collateral received from the borrowers of the securities, resulting in losses associated with the 2008 financial crisis that they seek to recover. The cases assert various contractual, statutory and common law claims, including claims for breach of fiduciary duty under common law and under the Employee Retirement Income Security Act (ERISA). In 2013, Northern Trust recorded a $19.2 million charge in connection with a settlement to resolve certain claims related to two of these lawsuits. In the second quarter of 2016, Northern Trust recorded a $46.5 million charge in connection with a settlement to resolve substantially all of the remaining claims. This settlement is not final as it requires further documentation, signed agreements and court approval. One individual lawsuit related to this matter is not subject to the settlement and remains pending.

As previously disclosed in April 2014, Northern Trust remains subject to an investigation by the U.S. Securities and Exchange Commission (SEC) related to Northern Trust’s securities lending activities. Northern Trust continues to cooperate with the SEC in this investigation.

In January 2015, the Public Prosecutor’s Office of France recommended that certain charges be brought against Northern Trust Fiduciary Services (Guernsey) Limited (NTFS), an indirect subsidiary of the Corporation, relating to the administration of two trusts for which NTFS serves as trustee. In April 2015, a French investigating magistrate judge charged NTFS with complicity in estate tax fraud. Charges also were brought against a number of other persons and entities related to this matter. As trustee, NTFS provided no tax advice and had no involvement in the preparation or filing of the challenged estate tax filings. NTFS has contested the criminal charge in the French court. After being postponed by the trial court in January 2016, the trial currently is scheduled for September 2016.

Visa Class B Common Shares. Northern Trust, as a member of Visa U.S.A. Inc. (Visa U.S.A.) and in connection with the 2007 restructuring of Visa U.S.A. and its affiliates and the 2008 initial public offering of Visa Inc. (Visa), received certain Visa Class B common shares. The Visa Class B common shares are subject to certain selling restrictions until the final resolution of the covered litigation noted below, at which time the shares are convertible into Visa Class A common shares based on a conversion rate dependent upon the ultimate cost of resolving the covered litigation.

Certain members of Visa U.S.A. are obligated to indemnify Visa for losses resulting from certain litigation relating to interchange fees (the covered litigation). On October 19, 2012, Visa signed a settlement agreement with plaintiff representatives for binding settlement of the covered litigation. On January 14, 2014, the United States District Court for the Eastern District of New York entered a final judgment order approving the settlement with the class plaintiffs. A number of objectors appealed from that order and more than 30 opt-out cases have been filed by merchants in various federal district courts. On June 30, 2016, the United States Court of Appeals for the Second Circuit reversed the District Court’s approval of the settlement and remanded the case to the District Court for further proceedings. The ultimate resolution of the covered litigation and the timing for removal of the selling restrictions on the Visa Class B common shares are uncertain.

Northern Trust recorded a $123.1 million and $99.9 million net gain on the sale of 1.1 million and 1.0 million of its Visa Class B common shares for the three months ended June 30, 2016 and 2015, respectively. Northern Trust continued to hold approximately 4.1 million and 5.2 million Visa Class B common shares, which are recorded at their original cost basis of zero as of June 30, 2016 and 2015, respectively.