Pledged Assets	3 Months Ended
Mar. 31, 2018
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, for potential Federal Reserve Bank discount window borrowings, and for derivative contracts. As of March 31, 2018, securities and loans totaling $40.8 billion ($31.9 billion of government-sponsored agency and other securities, $708.4 million of obligations of states and political subdivisions and $8.2 billion of loans) were pledged. This compares to $40.1 billion ($30.8 billion of government-sponsored agency and other securities, $684.3 million of obligations of states and political subdivisions and $8.6 billion of loans) at December 31, 2017. Collateral required for these purposes totaled $11.3 billion and $8.9 billion at March 31, 2018 and December 31, 2017, respectively. Available for sale debt securities with a total fair value of $785.0 million and $833.4 million, as of March 31, 2018 and December 31, 2017, respectively, were included in the total pledged assets, which were pledged as collateral for agreements to repurchase securities sold transactions and derivative contracts. 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 securities accepted as collateral under certain repurchase agreements. The total fair value of securities accepted as collateral was $1.4 billion as of March 31, 2018 and $1.2 billion as of December 31, 2017. Northern Trust has the right to repledge or sell securities accepted as collateral under certain repurchase agreements. The fair value of these securities accepted as collateral was $152.1 million as of March 31, 2018 and $78.3 million as of December 31, 2017. There was no repledged or sold collateral at March 31, 2018 or December 31, 2017. Northern Trust has the right to repledge or sell securities accepted as collateral under derivative contracts. The total fair value of securities accepted as collateral was $22.8 million as of March 31, 2018 and $4.6 million as of December 31, 2017. There was no repledged or sold collateral at March 31, 2018 or December 31, 2017. Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $1.8 billion and $2.0 billion for the three months ended March 31, 2018 and 2017, 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, for potential Federal Reserve Bank discount window borrowings, and for derivative contracts. As of March 31, 2018, securities and loans totaling $40.8 billion ($31.9 billion of government-sponsored agency and other securities, $708.4 million of obligations of states and political subdivisions and $8.2 billion of loans) were pledged. This compares to $40.1 billion ($30.8 billion of government-sponsored agency and other securities, $684.3 million of obligations of states and political subdivisions and $8.6 billion of loans) at December 31, 2017. Collateral required for these purposes totaled $11.3 billion and $8.9 billion at March 31, 2018 and December 31, 2017, respectively. Available for sale debt securities with a total fair value of $785.0 million and $833.4 million, as of March 31, 2018 and December 31, 2017, respectively, were included in the total pledged assets, which were pledged as collateral for agreements to repurchase securities sold transactions and derivative contracts. 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 securities accepted as collateral under certain repurchase agreements. The total fair value of securities accepted as collateral was $1.4 billion as of March 31, 2018 and $1.2 billion as of December 31, 2017.

Northern Trust has the right to repledge or sell securities accepted as collateral under certain repurchase agreements. The fair value of these securities accepted as collateral was $152.1 million as of March 31, 2018 and $78.3 million as of December 31, 2017. There was no repledged or sold collateral at March 31, 2018 or December 31, 2017.

Northern Trust has the right to repledge or sell securities accepted as collateral under derivative contracts. The total fair value of securities accepted as collateral was $22.8 million as of March 31, 2018 and $4.6 million as of December 31, 2017. There was no repledged or sold collateral at March 31, 2018 or December 31, 2017.

Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $1.8 billion and $2.0 billion for the three months ended March 31, 2018 and 2017, respectively.