Pledged Assets	3 Months Ended
Mar. 31, 2016
Transfers and Servicing [Abstract]
Pledged Assets	Pledged Assets — Certain of Northern Trust’s subsidiaries, as required or permitted by law, pledge assets to secure public and trust deposits, repurchase agreements and Federal Home Loan Bank borrowings, as well as for other purposes, including support for securities settlement, primarily related to client activities, and for potential Federal Reserve Bank discount window borrowings. As of March 31, 2016, securities and loans totaling $35.5 billion ($26.8 billion of government-sponsored agency and other securities, $211.0 million of obligations of states and political subdivisions and $8.5 billion of loans) were pledged. This compares to $35.8 billion ($27.1 billion of government-sponsored agency and other securities, $117.5 million of obligations of states and political subdivisions and $8.6 billion of loans) at December 31, 2015. Collateral required for these purposes totaled $6.9 billion and $8.9 billion at March 31, 2016 and December 31, 2015, respectively. Included in the total pledged assets at March 31, 2016 and December 31, 2015, were available for sale securities with a total fair value of $1,405.7 million and $1,545.6 million, respectively, which were pledged as collateral for agreements to repurchase securities sold transactions. The secured parties to these transactions have the right to repledge or sell these securities. Northern Trust is not permitted, by contract or custom, to repledge or sell collateral from agreements to resell securities purchased transactions. The total fair value of accepted collateral was $1.6 billion as of March 31, 2016 and $1.5 billion as of December 31, 2015. There was no repledged or sold collateral at March 31, 2016 or December 31, 2015. Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $1.8 billion and $1.5 billion for the three months ended March 31, 2016 and 2015, respectively.

Pledged Assets — Certain of Northern Trust’s subsidiaries, as required or permitted by law, pledge assets to secure public and trust deposits, repurchase agreements and Federal Home Loan Bank borrowings, as well as for other purposes, including support for securities settlement, primarily related to client activities, and for potential Federal Reserve Bank discount window borrowings. As of March 31, 2016, securities and loans totaling $35.5 billion ($26.8 billion of government-sponsored agency and other securities, $211.0 million of obligations of states and political subdivisions and $8.5 billion of loans) were pledged. This compares to $35.8 billion ($27.1 billion of government-sponsored agency and other securities, $117.5 million of obligations of states and political subdivisions and $8.6 billion of loans) at December 31, 2015. Collateral required for these purposes totaled $6.9 billion and $8.9 billion at March 31, 2016 and December 31, 2015, respectively. Included in the total pledged assets at March 31, 2016 and December 31, 2015, were available for sale securities with a total fair value of $1,405.7 million and $1,545.6 million, respectively, which were pledged as collateral for agreements to repurchase securities sold transactions. The secured parties to these transactions have the right to repledge or sell these securities.

Northern Trust is not permitted, by contract or custom, to repledge or sell collateral from agreements to resell securities purchased transactions. The total fair value of accepted collateral was $1.6 billion as of March 31, 2016 and $1.5 billion as of December 31, 2015. There was no repledged or sold collateral at March 31, 2016 or December 31, 2015.

Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $1.8 billion and $1.5 billion for the three months ended March 31, 2016 and 2015, respectively.