Pledged and Restricted Assets	12 Months Ended
Dec. 31, 2017
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, 2017, securities and loans totaling $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) were pledged. This compares to $38.9 billion ($28.3 billion of government-sponsored agency and other securities, $939.8 million of obligations of states and political subdivisions and $9.6 billion of loans) at December 31, 2016. Collateral required for these purposes totaled $11.0 billion and $9.3 billion at December 31, 2017 and December 31, 2016, respectively. Available for sale securities with a total fair value of $833.4 million and $494.7 million, as of December 31, 2017 and December 31, 2016, 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 $1.2 billion as of December 31, 2017 and $1.8 billion as of December 31, 2016. 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 $78.3 million as of December 31, 2017 and $217.5 million as of December 31, 2016. There was no repledged or sold collateral as of December 31, 2017 or December 31, 2016. 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 $4.6 million as of December 31, 2017. There were no securities accepted as collateral under derivative contracts as of December 31, 2016. Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $3.1 billion in 2017 as compared to $2.2 billion in 2016.

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, 2017, securities and loans totaling $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) were pledged. This compares to $38.9 billion ($28.3 billion of government-sponsored agency and other securities, $939.8 million of obligations of states and political subdivisions and $9.6 billion of loans) at December 31, 2016. Collateral required for these purposes totaled $11.0 billion and $9.3 billion at December 31, 2017 and December 31, 2016, respectively. Available for sale securities with a total fair value of $833.4 million and $494.7 million, as of December 31, 2017 and December 31, 2016, 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 $1.2 billion as of December 31, 2017 and $1.8 billion as of December 31, 2016.

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 $78.3 million as of December 31, 2017 and $217.5 million as of December 31, 2016. There was no repledged or sold collateral as of December 31, 2017 or December 31, 2016.

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 $4.6 million as of December 31, 2017. There were no securities accepted as collateral under derivative contracts as of December 31, 2016.

Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $3.1 billion in 2017 as compared to $2.2 billion in 2016.