Variable Interest Entities (Policy)	9 Months Ended
Variable Interest Entities (Policy)	Sep. 30, 2011
Variable Interest Entities
Asset-Backed Securitization Trusts	We are involved with various types of variable interest entities, or VIEs, as defined by GAAP, some of which are recorded in our consolidated financial statements and all of which are described below. We also invest in various forms of asset-backed securities, which we carry in our investment securities portfolio. These asset-backed securities meet the GAAP definition of asset securitization entities, which entities are considered to be VIEs. We are not considered to be the primary beneficiary of these VIEs, as defined by GAAP, since we do not have control over their activities. Additional information about our asset-backed securities is provided in note 3.
Tax-Exempt Investment Program	*Tax-Exempt Investment Program* In the normal course of our business, we structure and sell certificated interests in pools of tax-exempt investment-grade assets, principally to our mutual fund clients. We structure these pools as partnership trusts, and the assets and liabilities of the trusts are recorded in our consolidated statement of condition as investment securities available for sale and other short-term borrowings. We may also provide liquidity and re-marketing services to the trusts. As of both September 30, 2011 and December 31, 2010, we carried investment securities available for sale, composed of securities related to state and political subdivisions, with a fair value of $2.85 billion, and as of the same dates we carried other short-term borrowings of $2.38 billion and $2.50 billion, respectively, in our consolidated statement of condition in connection with these trusts. We transfer assets to the trusts from our investment securities portfolio at adjusted book value, and the trusts finance the acquisition of these assets by selling certificated interests issued by the trusts to third-party investors and to State Street as residual holder. These transfers do not meet the de-recognition criteria defined by GAAP, and therefore are recorded in our consolidated financial statements. The trusts had a weighted-average life of approximately 7.4 years at September 30, 2011, compared to approximately 7.7 years at December 31, 2010. Under separate legal agreements, we provide standby bond-purchase agreements to these trusts, which obligate State Street to acquire the certificated interests at par value in the event that the re-marketing agent is unable to place the certificated interests with investors. Our obligations as provider of the standby bond-purchase agreements terminate in the event of the following credit events: payment default, bankruptcy of the issuer and the credit enhancer, if any, the imposition of taxability, or the downgrade of an asset held by the trust below investment grade. Our commitments to the trusts under these standby bond-purchase agreements totaled $2.42 billion at September 30, 2011, none of which was utilized at period-end. In the event that our obligations under these agreements are triggered, no material impact to our consolidated results of operations or financial condition is expected to occur, because the securities are already recorded at fair value in our consolidated statement of condition.
Asset-Backed Commercial Paper Program	*Asset-Backed Commercial Paper Program* We sponsor and administer multi-seller asset-backed commercial paper programs, or conduits, which are recorded in our consolidated financial statements. These conduits were originally designed to satisfy the demand of our institutional clients, particularly mutual fund clients, for commercial paper. The conduits purchase financial assets with various asset classifications from a variety of independent third parties, and we consider the activities of the conduits in our liquidity management process. The conduits hold diversified investments, which are primarily asset-backed securities purchased from independent third parties, collateralized by student loans, automobile and equipment loans and credit card receivables, among other asset types. As of September 30, 2011 and December 31, 2010, we carried investment securities, composed primarily of asset-backed securities, with an aggregate carrying value of $294 million and $5.01 billion, respectively, and loans, composed of purchased receivables, with a recorded investment of $1.08 billion and $2.20 billion, respectively, in our consolidated statement of condition in connection with the conduits. In addition, as of December 31, 2010 we carried aggregate short-term borrowings, composed of commercial paper, of $1.92 billion in connection with the conduits. There was no commercial paper outstanding to third parties as of September 30, 2011 associated with the conduits.
Collateralized Debt Obligations	*Collateralized Debt Obligations* We serve as collateral manager for a series of collateralized debt obligations, referred to as CDOs. A CDO is a structured investment vehicle which purchases a portfolio of assets funded through the issuance of several classes of debt and equity, the repayment of and return on which are linked to the performance of the underlying assets. We have determined that we are not the primary beneficiary of these VIEs, and do not record them in our consolidated financial statements. At both September 30, 2011 and December 31, 2010, the aggregate notional amount of these CDOs was $1.0 billion. At September 30, 2011 and December 31, 2010, the carrying amount of the underlying collateral was $275 million and $323 million, respectively. We have not acquired or transferred any investment securities to a CDO since 2005.