Variable Interest Entities	12 Months Ended
Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Variable Interest Entities	Variable Interest Entities Variable Interest Entities (VIEs) are defined within GAAP as entities which either have a total equity investment that is insufficient to permit the entity to finance its activities without additional subordinated financial support or whose equity investors lack the characteristics of a controlling financial interest. Investors that finance a VIE through debt or equity interests, or other counterparties that provide other forms of support, such as guarantees, subordinated fee arrangements, or certain types of derivative contracts, are variable interest holders in the entity and the variable interest holder, if any, that has both the power to direct the activities that most significantly impact the entity and a variable interest that could potentially be significant to the entity is deemed to be the VIE’s primary beneficiary and is required to consolidate the VIE. Leveraged Leases. In leveraged leasing transactions, Northern Trust acts as lessor of the underlying asset subject to the lease and typically funds 20-30% of the asset’s cost via an equity ownership in a trust with the remaining 70-80% provided by third party non-recourse debt holders. In such transactions, the trusts, which are VIEs, are created to provide the lessee use of the property with substantially all of the rights and obligations of ownership. The lessee’s maintenance and operation of the leased property has a direct effect on the fair value of the underlying property, and the lessee also has the ability to increase the benefits it can receive and limit the losses it can suffer by the manner in which it uses the property. As a result, Northern Trust has determined that it is not the primary beneficiary of these VIEs given it lacks the power to direct the activities that most significantly impact the economic performance of the VIEs. Northern Trust’s maximum exposure to loss as a result of its involvement with the leveraged lease trust VIEs is limited to the carrying amounts of its leveraged lease investments. As of December 31, 2015 and 2014, the carrying amounts of these investments, which are included in loans and leases in the consolidated balance sheets, were $389.4 million and $547.6 million, respectively. Northern Trust’s funding requirements relative to the VIEs are limited to its invested capital. Northern Trust has no other liquidity arrangements or obligations to purchase assets of the VIEs that would expose Northern Trust to a loss. Tax Credit Structures. Northern Trust invests in qualified affordable housing projects and community development entities (collectively, community development projects) that are designed to generate a return primarily through the realization of tax credits. The community development projects are formed as limited partnerships and LLCs in which Northern Trust invests as a limited partner/investor member through equity contributions. The economic performance of the community development projects, which are VIEs, is subject to the performance of their underlying investment and their ability to operate in compliance with the rules and regulations necessary for the qualification of tax credits generated by equity investments. Northern Trust has determined that it is not the primary beneficiary of any community development projects as it lacks the power to direct the activities that most significantly impact the economic performance of the underlying investments or to affect their ability to operate in compliance with the rules and regulations necessary for the qualification of tax credits generated by equity investments. This power is held by the general partners and managing members who exercise full and exclusive control of the operations of the VIEs. Northern Trust’s maximum exposure to loss as a result of its involvement with community development projects is limited to the carrying amounts of its investments, including any undrawn commitments. As of December 31, 2015 and 2014, the carrying amounts of these investments in community development projects that generate tax credits, included in other assets in the consolidated balance sheets, were $173.5 million and $208.9 million, respectively. As of December 31, 2015 and 2014, liabilities related to unfunded commitments on investments in tax credit community development projects, included in other liabilities in the consolidated balance sheets, were $10.5 million and $15.6 million, respectively. Northern Trust’s funding requirements are limited to its invested capital and unfunded commitments for future equity contributions. Northern Trust has no exposure to loss from liquidity arrangements and no obligation to purchase assets of the community development projects. Affordable housing tax credits and other tax benefits attributable to community development projects totaled $52.5 million and $58.1 million, respectively, as of December 31, 2015 and 2014. Investment Funds. Northern Trust acts as asset manager for various funds in which clients of Northern Trust are investors. As an asset manager of funds, the Corporation earns a competitively priced fee that is based on assets managed and varies with each fund’s investment objective. Periodically, Northern Trust makes seed capital investments to certain funds. As of December 31, 2015 Northern Trust had a $25.3 million investment valued using net asset value per share, included in other assets, and a $25.0 million unfunded commitment related to seed capital investments. In June 2015, Northern Trust voluntarily elected to contribute an aggregate $45.8 million of cash to four constant-dollar-net-asset-value investment funds (Funds) for which it serves as asset manager to bring the net asset values of such funds to $1.00. The contributions, which were recorded to other operating expense in the consolidated statements of income, resulted in a pre-tax charge of $45.8 million. Under GAAP, the contributions noted above are deemed to reflect Northern Trust’s implicit interest in the credit risk of the Funds, which must be considered when determining whether it is the primary beneficiary of the Funds. In determining whether Northern Trust is the primary beneficiary of the Funds, Northern Trust used an expected loss calculation based on the characteristics of the underlying investments in the Funds to estimate the expected losses related to interest rate and credit risk, and also considered the relative rights and obligations of each of the applicable variable interest holders. Upon consideration of this analysis, Northern Trust determined that it is not the primary beneficiary of the Funds, as interest rate risk was determined to be the primary driver of expected losses within such funds. Similarly, based on its analysis, including consideration of the contributions noted above, Northern Trust has also determined that it is not the primary beneficiary of any other investment funds for which it serves as asset manager under GAAP. Accordingly, Northern Trust is not required to consolidate the Funds or any other investment funds for which it serves as asset manager within its consolidated balance sheets. Any potential future support of the Funds will be at the discretion of Northern Trust after an evaluation of the specific facts and circumstances and with careful consideration as to the potential impact on Northern Trust’s regulatory capital levels and other operational needs. As of December 31, 2015, Northern Trust has no exposure to loss from its implicit interest in such Funds as there are no current plans to provide any further support to the Funds.

Variable Interest Entities

Variable Interest Entities (VIEs) are defined within GAAP as entities which either have a total equity investment that is insufficient to permit the entity to finance its activities without additional subordinated financial support or whose equity investors lack the characteristics of a controlling financial interest. Investors that finance a VIE through debt or equity interests, or other counterparties that provide other forms of support, such as guarantees, subordinated fee arrangements, or certain types of derivative contracts, are variable interest holders in the entity and the variable interest holder, if any, that has both the power to direct the activities that most significantly impact the entity and a variable interest that could potentially be significant to the entity is deemed to be the VIE’s primary beneficiary and is required to consolidate the VIE.

Leveraged Leases. In leveraged leasing transactions, Northern Trust acts as lessor of the underlying asset subject to the lease and typically funds 20-30% of the asset’s cost via an equity ownership in a trust with the remaining 70-80% provided by third party non-recourse debt holders. In such transactions, the trusts, which are VIEs, are created to provide the lessee use of the property with substantially all of the rights and obligations of ownership. The lessee’s maintenance and operation of the leased property has a direct effect on the fair value of the underlying property, and the lessee also has the ability to increase the benefits it can receive and limit the losses it can suffer by the manner in which it uses the property. As a result, Northern Trust has determined that it is not the primary beneficiary of these VIEs given it lacks the power to direct the activities that most significantly impact the economic performance of the VIEs.

Northern Trust’s maximum exposure to loss as a result of its involvement with the leveraged lease trust VIEs is limited to the carrying amounts of its leveraged lease investments. As of December 31, 2015 and 2014, the carrying amounts of these investments, which are included in loans and leases in the consolidated balance sheets, were $389.4 million and $547.6 million, respectively. Northern Trust’s funding requirements relative to the VIEs are limited to its invested capital. Northern Trust has no other liquidity arrangements or obligations to purchase assets of the VIEs that would expose Northern Trust to a loss.

Tax Credit Structures. Northern Trust invests in qualified affordable housing projects and community development entities (collectively, community development projects) that are designed to generate a return primarily through the realization of tax credits. The community development projects are formed as limited partnerships and LLCs in which Northern Trust invests as a limited partner/investor member through equity contributions. The economic performance of the community development projects, which are VIEs, is subject to the performance of their underlying investment and their ability to operate in compliance with the rules and regulations necessary for the qualification of tax credits generated by equity investments. Northern Trust has determined that it is not the primary beneficiary of any community development projects as it lacks the power to direct the activities that most significantly impact the economic performance of the underlying investments or to affect their ability to operate in compliance with the rules and regulations necessary for the qualification of tax credits generated by equity investments. This power is held by the general partners and managing members who exercise full and exclusive control of the operations of the VIEs.

Northern Trust’s maximum exposure to loss as a result of its involvement with community development projects is limited to the carrying amounts of its investments, including any undrawn commitments. As of December 31, 2015 and 2014, the carrying amounts of these investments in community development projects that generate tax credits, included in other assets in the consolidated balance sheets, were $173.5 million and $208.9 million, respectively. As of December 31, 2015 and 2014, liabilities related to unfunded commitments on investments in tax credit community development projects, included in other liabilities in the consolidated balance sheets, were $10.5 million and $15.6 million, respectively. Northern Trust’s funding requirements are limited to its invested capital and unfunded commitments for future equity contributions. Northern Trust has no exposure to loss from liquidity arrangements and no obligation to purchase assets of the community development projects.

Affordable housing tax credits and other tax benefits attributable to community development projects totaled $52.5 million and $58.1 million, respectively, as of December 31, 2015 and 2014.

Investment Funds. Northern Trust acts as asset manager for various funds in which clients of Northern Trust are investors. As an asset manager of funds, the Corporation earns a competitively priced fee that is based on assets managed and varies with each fund’s investment objective. Periodically, Northern Trust makes seed capital investments to certain funds. As of December 31, 2015 Northern Trust had a $25.3 million investment valued using net asset value per share, included in other assets, and a $25.0 million unfunded commitment related to seed capital investments.

In June 2015, Northern Trust voluntarily elected to contribute an aggregate $45.8 million of cash to four constant-dollar-net-asset-value investment funds (Funds) for which it serves as asset manager to bring the net asset values of such funds to $1.00. The contributions, which were recorded to other operating expense in the consolidated statements of income, resulted in a pre-tax charge of $45.8 million. Under GAAP, the contributions noted above are deemed to reflect Northern Trust’s implicit interest in the credit risk of the Funds, which must be considered when determining whether it is the primary beneficiary of the Funds. In determining whether Northern Trust is the primary beneficiary of the Funds, Northern Trust used an expected loss calculation based on the characteristics of the underlying investments in the Funds to estimate the expected losses related to interest rate and credit risk, and also considered the relative rights and obligations of each of the applicable variable interest holders. Upon consideration of this analysis, Northern Trust determined that it is not the primary beneficiary of the Funds, as interest rate risk was determined to be the primary driver of expected losses within such funds. Similarly, based on its analysis, including consideration of the contributions noted above, Northern Trust has also determined that it is not the primary beneficiary of any other investment funds for which it serves as asset manager under GAAP. Accordingly, Northern Trust is not required to consolidate the Funds or any other investment funds for which it serves as asset manager within its consolidated balance sheets. Any potential future support of the Funds will be at the discretion of Northern Trust after an evaluation of the specific facts and circumstances and with careful consideration as to the potential impact on Northern Trust’s regulatory capital levels and other operational needs. As of December 31, 2015, Northern Trust has no exposure to loss from its implicit interest in such Funds as there are no current plans to provide any further support to the Funds.