Stock Index Fund Investment Strategy - Stock Index Fund	Dec. 31, 2024
Prospectus [Line Items]
Strategy [Heading]	<span style="color:#39A872;font-family:Arial;font-size:10pt;font-weight:bold;">Principal Investment Strategies</span>
Strategy Narrative [Text Block]	The Stock Index Fund pursues its investment objective by seeking to replicate the total return performance of the Index, which is composed of 500 selected common stocks, most of which are listed on the New York Stock Exchange (the “NYSE”).The Stock Index Fund is a feeder fund, meaning that it invests all of its investable assets in a master portfolio. The Fund invests its assets in the S&P 500 Index Master Portfolio (“Master Portfolio”), a separate series of an unaffiliated trust called the Master Investment Portfolio. The Master Portfolio and the Stock Index Fund have substantially similar investment objectives and investment strategies. This structure is sometimes called a “master/feeder” structure. The Fund’s investment results will correspond directly to the investment results of the Master Portfolio.Under normal circumstances, at least 90% of the value of the Master Portfolio’s assets, plus the amount of any borrowing for investment purposes, is invested in securities comprising the Index. The Master Portfolio attempts to achieve, in both rising and falling markets, a correlation of at least 95% between the total return of its net assets before fees and expenses and the total return of the Index.The Master Portfolio seeks to replicate the total return performance of the Index by investing the Master Portfolio’s assets so that the percentage of assets of the Master Portfolio invested in a given stock is approximately the same as the percentage such stock represents in the Index. No attempt is made to manage the Master Portfolio using economic, financial or market analysis. In addition, at times, the portfolio composition of the Master Portfolio may be altered (or “rebalanced”) to reflect changes in the characteristics of the Index. The Index is normally rebalanced quarterly during the months of March, June, September and December.The Master Portfolio also may engage in futures transactions and other derivative securities transactions and lend its portfolio securities, each of which involves risk. The Master Portfolio may use futures contracts and other derivative transactions to manage its short-term liquidity and/or as substitutes for comparable market positions in the securities in its benchmark index. The Master Portfolio may also invest in high-quality money market instruments, including shares of money market funds advised by BlackRock Fund Advisors (“BFA”) or its affiliates.The Master Portfolio reserves the right to concentrate its investments (i.e., invest 25% or more of its total assets in securities of issuers in a particular industry) to approximately the same extent that its benchmark index concentrates in a particular industry.