Pledged and Restricted Assets	12 Months Ended
Dec. 31, 2018
Financial Instruments Owned and Pledged as Collateral [Abstract]
Pledged and Restricted Assets	Pledged and Restricted 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 December 31, 2018, securities and loans totaling $39.6 billion ($30.9 billion of government-sponsored agency and other securities, $640.4 million of obligations of states and political subdivisions and $8.1 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 $9.3 billion and $8.9 billion at December 31, 2018 and December 31, 2017, respectively. Included in the total pledged assets are available for sale debt securities with a total fair value of $151.5 million and $833.4 million, as of December 31, 2018 and December 31, 2017, respectively, which were pledged as collateral for agreements to repurchase securities sold transactions, and available for sale debt securities with a total fair value of $29.0 million and $39.9 million, as of December 31, 2018 and December 31, 2017, respectively, which were pledged as collateral for derivative contracts. The secured parties to these transactions have the right to repledge or sell the securities as it relates to $151.5 million and $833.4 million of the pledged collateral as of December 31, 2018 and December 31, 2017, respectively. 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 $605.0 million as of December 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 $426.2 million as of December 31, 2018 and $78.3 million as of December 31, 2017. There was no repledged or sold collateral as of December 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 $15.3 million as of December 31, 2018. There were $4.6 million securities accepted as collateral under derivative contracts as of December 31, 2017. Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $1.7 billion in 2018 as compared to $3.1 billion in 2017.

Pledged and Restricted 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 December 31, 2018, securities and loans totaling $39.6 billion ($30.9 billion of government-sponsored agency and other securities, $640.4 million of obligations of states and political subdivisions and $8.1 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 $9.3 billion and $8.9 billion at December 31, 2018 and December 31, 2017, respectively. Included in the total pledged assets are available for sale debt securities with a total fair value of $151.5 million and $833.4 million, as of December 31, 2018 and December 31, 2017, respectively, which were pledged as collateral for agreements to repurchase securities sold transactions, and available for sale debt securities with a total fair value of $29.0 million and $39.9 million, as of December 31, 2018 and December 31, 2017, respectively, which were pledged as collateral for derivative contracts. The secured parties to these transactions have the right to repledge or sell the securities as it relates to $151.5 million and $833.4 million of the pledged collateral as of December 31, 2018 and December 31, 2017, respectively.

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 $605.0 million as of December 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 $426.2 million as of December 31, 2018 and $78.3 million as of December 31, 2017. There was no repledged or sold collateral as of December 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 $15.3 million as of December 31, 2018. There were $4.6 million securities accepted as collateral under derivative contracts as of December 31, 2017.

Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $1.7 billion in 2018 as compared to $3.1 billion in 2017.