Contingencies	12 Months Ended
Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies Legal and Regulatory Matters In the ordinary course of business, we and our subsidiaries are involved in disputes, litigation, and governmental or regulatory inquiries and investigations, both pending and threatened. These matters, if resolved adversely against us or settled, may result in monetary damages, fines and penalties or require changes in our business practices. The resolution or settlement of these matters is inherently difficult to predict. Based on our assessment of these pending matters, we do not believe that the amount of any judgment, settlement or other action arising from any pending matter is likely to have a material adverse effect on our consolidated financial condition. However, an adverse outcome in certain of the matters described below could have a material adverse effect on our consolidated results of operations for the period in which such matter is resolved, or an accrual is determined to be required, on our consolidated financial condition, or on our reputation. We evaluate our needs for accruals of loss contingencies related to legal and regulatory proceedings on a case-by-case basis. When we have a liability that we deem probable, and we deem the amount of such liability can be reasonably estimated as of the date of our consolidated financial statements, we accrue our estimate of the amount of loss. We also consider a loss probable and establish an accrual when we make, or intend to make, an offer of settlement. Once established, an accrual is subject to subsequent adjustment as a result of additional information. The resolution of legal and regulatory proceedings and the amount of reasonably estimable loss (or range thereof) are inherently difficult to predict, especially in the early stages of proceedings. Even if a loss is probable, an amount (or range) of loss might not be reasonably estimated until the later stages of the proceeding due to many factors such as the presence of complex or novel legal theories, the discretion of governmental authorities in seeking sanctions or negotiating resolutions in civil and criminal matters, the pace and timing of discovery and other assessments of facts and the procedural posture of the matter (collectively, "factors influencing reasonable estimates"). As of December 31, 2017, our aggregate accruals for loss contingencies for legal and regulatory matters totaled approximately $12 million. To the extent that we have established accruals in our consolidated statement of condition for probable loss contingencies, such accruals may not be sufficient to cover our ultimate financial exposure associated with any settlements or judgments. Any such ultimate financial exposure, or proceedings to which we may become subject in the future, could have a material adverse effect on our businesses, on our future consolidated financial statements or on our reputation. Except where otherwise noted below, we have not established significant accruals with respect to the claims discussed. We have identified certain matters for which loss is reasonably possible (but not probable) in future periods, whether in excess of an accrued liability or where there is no accrued liability, and for which we are able to estimate a range of reasonably possible loss. As of December 31, 2017, our estimate of the range of reasonably possible loss for these matters is from zero to approximately $15 million in the aggregate. Our estimate with respect to the aggregate range of reasonably possible loss is based upon currently available information and is subject to significant judgment and a variety of assumptions and known and unknown uncertainties. The matters underlying the estimated range will change from time to time, and actual results may vary significantly from the current estimate. In certain other pending matters, including the invoicing matter, Federal Reserve/Massachusetts Division of Banks written agreement and shareholder litigation matters discussed below, it is not currently feasible to reasonably estimate the amount or a range of reasonably possible loss (including reasonably possible loss in excess of amounts accrued), and such losses, which may be significant, are not included in the estimate of reasonably possible loss discussed above. This is due to, among other factors, the factors influencing reasonable estimates described above. These factors are particularly prevalent in governmental and regulatory inquiries and investigations. As a result, reasonably possible loss estimates often are not feasible until the later stages of the inquiry or investigation or of any related legal or regulatory proceeding. An adverse outcome in one or more of the matters for which we have not estimated the amount or a range of reasonably possible loss, individually or in the aggregate, could have a material adverse effect on our businesses, on our future consolidated financial statements or on our reputation. Given that we cannot estimate reasonably possible loss for all legal and regulatory proceedings as to which we may be subject now or in the future, including matters that if adversely concluded may present material financial, regulatory and reputational risks, no conclusion as to the ultimate exposure from current pending or potential legal or regulatory proceedings should be drawn from the current estimate of reasonably possible loss. The following discussion provides information with respect to significant legal, governmental and regulatory matters. Invoicing Matter In December 2015, we announced a review of the manner in which we invoiced certain expenses to some of our Investment Servicing clients, primarily in the United States, during an 18-year period going back to 1998, and our determination that we had incorrectly invoiced clients for certain expenses. We informed our clients in December 2015 that we will pay to them the amounts we concluded were incorrectly invoiced to them, plus interest. We are implementing enhancements to our billing processes, and we are reviewing the conduct of our employees and have taken appropriate steps to address conduct inconsistent with our standards, including, in some cases, termination of employment. In connection with our enhancements to our billing processes, we continue to review historical billing practices and may from time to time identify additional remediation. We currently expect the cumulative total of our payments to customers for these matters to be at least $360 million. We have received a purported class action demand letter alleging that our invoicing practices were unfair and deceptive under Massachusetts law. A class of customers, or particular customers, may assert that we have not paid to them all amounts incorrectly invoiced, and may seek double or treble damages under Massachusetts law. In addition, in March 2017, a purported class action was commenced against us alleging that our invoicing practices violated duties owed to retirement plan customers under ERISA. We are also responding to requests for information from, and are cooperating with investigations by, governmental and regulatory authorities on these matters, including the civil and criminal divisions of the DOJ, the SEC, the DOL, the Massachusetts Attorney General, and the New Hampshire Bureau of Securities Regulation, which could result in significant fines or other sanctions, civil and criminal, against us. If these governmental or regulatory authorities were to conclude that all or a portion of the billing errors merited civil or criminal sanctions, any fine or other penalty could be a significant percentage, or a multiple of, the portion of the overcharging serving as the basis of such a claim or of the full amount overcharged. The governmental and regulatory authorities have significant discretion in civil and criminal matters as to the fines and other penalties they may seek to impose. The severity of such fines or other penalties could take into account factors such as the amount and duration of our incorrect invoicing, the government’s or regulator's assessment of the conduct of our employees, as well as prior conduct such as that which resulted in our January 2017 deferred prosecution agreement in connection with transition management services and our recent settlement of civil claims regarding our indirect foreign exchange business. The outcome of any of these proceedings and, in particular, any criminal sanction could materially adversely affect our results of operations and could have significant collateral consequences for our business and reputation. Federal Reserve/Massachusetts Division of Banks Written Agreement On June 1, 2015, we entered into a written agreement with the Federal Reserve and the Massachusetts Division of Banks relating to deficiencies identified in our compliance programs with the requirements of the Bank Secrecy Act, AML regulations and U.S. economic sanctions regulations promulgated by OFAC. As part of this enforcement action, we have been required to, among other things, implement improvements to our compliance programs and to retain an independent firm to conduct a review of account and transaction activity to evaluate whether any suspicious activity was not previously reported. If we fail to comply with the terms of the written agreement, we may become subject to fines and other regulatory sanctions, which may have a material adverse effect on us. Shareholder Litigation A State Street shareholder has filed a purported class action complaint against the Company alleging that the Company’s financial statements in its annual reports for the 2011-2014 period were misleading due to the inclusion of revenues associated with the invoicing matter referenced above and the facts surrounding our 2017 settlements with the U.S. government relating to our transition management business. In addition, a State Street shareholder has filed a derivative complaint against the Company's past and present officers and directors to recover alleged losses incurred by the Company relating to the invoicing matter and to our Ohio public retirement plans matter. Income Taxes In determining our provision for income taxes, we make certain judgments and interpretations with respect to tax laws in jurisdictions in which we have business operations. Because of the complex nature of these laws, in the normal course of our business, we are subject to challenges from U.S. and non-U.S. income tax authorities regarding the amount of income taxes due. These challenges may result in adjustments to the timing or amount of taxable income or deductions or the allocation of taxable income among tax jurisdictions. We recognize a tax benefit when it is more likely than not that our position will result in a tax deduction or credit. Additional information with respect to our provision for income taxes and tax benefits, including unrecognized tax benefits, is provided in Note 22. We are presently under audit by a number of tax authorities, and the Internal Revenue Service is currently reviewing our U.S. income tax returns for the tax years 2014 and 2015. The earliest tax year open to examination in jurisdictions where we have material operations is 2011. Management believes that we have sufficiently accrued liabilities as of December 31, 2017 for tax exposures.

Legal and Regulatory Matters

In the ordinary course of business, we and our subsidiaries are involved in disputes, litigation, and governmental or regulatory inquiries and investigations, both pending and threatened. These matters, if resolved adversely against us or settled, may result in monetary damages, fines and penalties or require changes in our business practices. The resolution or settlement of these matters is inherently difficult to predict. Based on our assessment of these pending matters, we do not believe that the amount of any judgment, settlement or other action arising from any pending matter is likely to have a material adverse effect on our consolidated financial condition. However, an adverse outcome in certain of the matters described below could have a material adverse effect on our consolidated results of operations for the period in which such matter is resolved, or an accrual is determined to be required, on our consolidated financial condition, or on our reputation.

We evaluate our needs for accruals of loss contingencies related to legal and regulatory proceedings on a case-by-case basis. When we have a liability that we deem probable, and we deem the amount of such liability can be reasonably estimated as of the date of our consolidated financial statements, we accrue our estimate of the amount of loss. We also consider a loss probable and establish an accrual when we make, or intend to make, an offer of settlement. Once established, an accrual is subject to subsequent adjustment as a result of additional information. The resolution of legal and regulatory proceedings and the amount of reasonably estimable loss (or range thereof) are inherently difficult to predict, especially in the early stages of proceedings. Even if a loss is probable, an amount (or range) of loss might not be reasonably estimated until the later stages of the proceeding due to many factors such as the presence of complex or novel legal theories, the discretion of governmental authorities in seeking sanctions or negotiating resolutions in civil and criminal matters, the pace and timing of discovery and other assessments of facts and the procedural posture of the matter (collectively, "factors influencing reasonable estimates").

As of December 31, 2017, our aggregate accruals for loss contingencies for legal and regulatory matters totaled approximately $12 million. To the extent that we have established accruals in our consolidated statement of condition for probable loss contingencies, such accruals may not be sufficient to cover our ultimate financial exposure associated with any settlements or judgments. Any such ultimate financial exposure, or proceedings to which we may become subject in the future, could have a material adverse effect on our businesses, on our future consolidated financial statements or on our reputation. Except where otherwise noted below, we have not established significant accruals with respect to the claims discussed.

We have identified certain matters for which loss is reasonably possible (but not probable) in future periods, whether in excess of an accrued liability or where there is no accrued liability, and for which we are able to estimate a range of reasonably possible loss. As of December 31, 2017, our estimate of the range of reasonably possible loss for these matters is from zero to approximately $15 million in the aggregate. Our estimate with respect to the aggregate range of reasonably possible loss is based upon currently available information and is subject to significant judgment and a variety of assumptions and known and unknown uncertainties. The matters underlying the estimated range will change from time to time, and actual results may vary significantly from the current estimate.

In certain other pending matters, including the invoicing matter, Federal Reserve/Massachusetts Division of Banks written agreement and shareholder litigation matters discussed below, it is not currently feasible to reasonably estimate the amount or a range of reasonably possible loss (including reasonably possible loss in excess of amounts accrued), and such losses, which may be significant, are not included in the estimate of reasonably possible loss discussed above. This is due to, among other factors, the factors influencing reasonable estimates described above. These factors are particularly prevalent in governmental and regulatory inquiries and investigations. As a result, reasonably possible loss estimates often are not feasible until the later stages of the inquiry or investigation or of any related legal or regulatory proceeding. An adverse outcome in one or more of the matters for which we have not estimated the amount or a range of reasonably possible loss, individually or in the aggregate, could have a material adverse effect on our businesses, on our future consolidated financial statements or on our reputation. Given that we cannot estimate reasonably possible loss for all legal and regulatory proceedings as to which we may be subject now or in the future, including matters that if adversely concluded may present material financial, regulatory and reputational risks, no conclusion as to the ultimate exposure from current pending or potential legal or regulatory proceedings should be drawn from the current estimate of reasonably possible loss.

The following discussion provides information with respect to significant legal, governmental and regulatory matters.

Invoicing Matter

In December 2015, we announced a review of the manner in which we invoiced certain expenses to some of our Investment Servicing clients, primarily in the United States, during an 18-year period going back to 1998, and our determination that we had incorrectly invoiced clients for certain expenses. We informed our clients in December 2015 that we will pay to them the amounts we concluded were incorrectly invoiced to them, plus interest. We are implementing enhancements to our billing processes, and we are reviewing the conduct of our employees and have taken appropriate steps to address conduct inconsistent with our standards, including, in some cases, termination of employment. In connection with our enhancements to our billing processes, we continue to review historical billing practices and may from time to time identify additional remediation. We currently expect the cumulative total of our payments to customers for these matters to be at least $360 million.

We have received a purported class action demand letter alleging that our invoicing practices were unfair and deceptive under Massachusetts law. A class of customers, or particular customers, may assert that we have not paid to them all amounts incorrectly invoiced, and may seek double or treble damages under Massachusetts law. In addition, in March 2017, a purported class action was commenced against us alleging that our invoicing practices violated duties owed to retirement plan customers under ERISA.

We are also responding to requests for information from, and are cooperating with investigations by, governmental and regulatory authorities on these matters, including the civil and criminal divisions of the DOJ, the SEC, the DOL, the Massachusetts Attorney General, and the New Hampshire Bureau of Securities Regulation, which could result in significant fines or other sanctions, civil and criminal, against us. If these governmental or regulatory authorities were to conclude that all or a portion of the billing errors merited civil or criminal sanctions, any fine or other penalty could be a significant percentage, or a multiple of, the portion of the overcharging serving as the basis of such a claim or of the full amount overcharged. The governmental and regulatory authorities have significant discretion in civil and criminal matters as to the fines and other penalties they may seek to impose. The severity of such fines or other penalties could take into account factors such as the amount and duration of our incorrect invoicing, the government’s or regulator's assessment of the conduct of our employees, as well as prior conduct such as that which resulted in our January 2017 deferred prosecution agreement in connection with transition management services and our recent settlement of civil claims regarding our indirect foreign exchange business.

The outcome of any of these proceedings and, in particular, any criminal sanction could materially adversely affect our results of operations and could have significant collateral consequences for our business and reputation.

Federal Reserve/Massachusetts Division of Banks Written Agreement

On June 1, 2015, we entered into a written agreement with the Federal Reserve and the Massachusetts Division of Banks relating to deficiencies identified in our compliance programs with the requirements of the Bank Secrecy Act, AML regulations and U.S. economic sanctions regulations promulgated by OFAC. As part of this enforcement action, we have been required to, among other things, implement improvements to our compliance programs and to retain an independent firm to conduct a review of account and transaction activity to evaluate whether any suspicious activity was not previously reported. If we fail to comply with the terms of the written agreement, we may become subject to fines and other regulatory sanctions, which may have a material adverse effect on us.

Shareholder Litigation

A State Street shareholder has filed a purported class action complaint against the Company alleging that the Company’s financial statements in its annual reports for the 2011-2014 period were misleading due to the inclusion of revenues associated with the invoicing matter referenced above and the facts surrounding our 2017 settlements with the U.S. government relating to our transition management business. In addition, a State Street shareholder has filed a derivative complaint against the Company's past and present officers and directors to recover alleged losses incurred by the Company relating to the invoicing matter and to our Ohio public retirement plans matter.