Objective, Primary [Text Block]

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

rr_ObjectivePrimaryTextBlock

The investment objective of the Short Term Investment Fund for Puerto Rico Residents, Inc. (the “Fund”) is to provide current income, consistent with liquidity and conservation of capital.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses of the Fund

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that you may pay if you buy, hold and sell shares of the Fund (“Shares”). You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

June 30, 2023

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover:

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Shares are held in a taxable

account. These costs, which are not reflected in annual fund operating expenses or in the Example above, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 0% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

none

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Total Annual Fund Operating Expenses have been restated to reflect current fees.

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then sell all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s total annual operating expenses remain the same. This assumption is not meant to indicate you will receive a 5% annual rate of return. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Principal Investment Strategies of the Fund

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund is designed solely for Puerto Rico Residents (as defined in “Dividends and Taxes” below). Only Puerto Rico Residents will receive the tax benefits of an investment in the Fund. The tax treatment of this Fund differs from that typically accorded to other investment companies registered under the Investment Company Act of 1940, as amended (the “1940 Act”) that are treated as regulated investment companies under Subchapter M of the Internal Revenue Code of 1986, as amended (the “U.S. Code”).

The Fund invests in a portfolio of high quality tax‑exempt short-term debt instruments of governmental and private issuers, including instruments issued by other Puerto Rico investment companies advised by or co‑advised by UBS Asset Managers of Puerto Rico (the “Investment Adviser”), the Fund’s investment adviser. “High quality” means instruments that at the time of investment are rated within the two highest short-term rating categories by one or more nationally recognized statistical rating organizations (“NRSRO”) or that are unrated but deemed to be of comparable quality by the Investment Adviser. The Fund may continue to hold these instruments even if their rating is downgraded. “Short-term” means instruments that mature in 397 calendar days or less from the date when the Fund acquires the instrument. The maturity of securities rated in the second highest short-term rating category by an NRSRO (“Second Tier Securities”) may not exceed 180 days. The Fund may also invest in longer-term bonds with only a short time remaining to maturity or that have variable interest rates or other special features that give them the financial characteristics of short-term debt. The Fund invests only in U.S. dollar-denominated instruments.

The Fund is not a money market fund and does not seek to maintain a stable net asset value of $1.00 per share.

Under normal circumstances, the Fund will invest at least 67% of its total assets in short-term securities of Puerto Rico issuers, including Puerto Rico municipal obligations, Puerto Rico mortgage-backed and asset-backed securities, tax‑free secured obligations (“TSOs”) of Puerto Rico investment companies (the majority of which will be obligations issued by investment companies for which, like the Fund, UBS Asset Managers of Puerto Rico acts as investment adviser or co‑investment adviser), repurchase agreements and commercial paper. While the Fund intends to comply with the above 67% investment requirement as market conditions permit, the Fund’s ability to procure sufficient Puerto Rico securities which meet the Fund’s investment criteria may, in the opinion of the Investment Adviser, be constrained, due to the volatility affecting the Puerto Rico bond market since 2013 and the fact that the Puerto Rico Government remains in the process of restructuring its outstanding debt under Title III of the Puerto Rico Oversight, Management, and Economic Stability Act (“PROMESA”) as well as undertaking other fiscal measures to stabilize Puerto Rico’s economy in accordance with the requirements of PROMESA, and this inability may continue for an indeterminate period of time. To the extent that the Fund is unable to procure sufficient amounts of such Puerto Rico securities, the Fund may acquire investments in securities of non‑Puerto Rico issuers which satisfy the Fund’s investment policies. The Fund will ensure that its investments in Puerto Rico securities constitute at least an average of 20% of its total assets on an annual basis.

The Fund may hold up to 33% of its total assets in U.S. high quality short-term instruments, including securities issued or guaranteed by the U.S. government, its agencies and instrumentalities, municipal securities of issuers in the United States and non‑Puerto Rico (i.e., the 50 States comprising the United States.) mortgage-backed and asset-backed securities.

Investments in mortgage-backed securities may include those issued or guaranteed by the Government National Mortgage Association (“GNMA”), Federal National Mortgage Association (“FNMA”) or Federal Home Loan Mortgage Corporation (“FHLMC”), as well as mortgage-backed securities that are not guaranteed or issued by GNMA, FNMA, FHLMC or any other government agency (“Private Label mortgage-backed securities”), and in either case may include collateralized mortgage obligations (“CMOs”). Private Label mortgage-backed securities are issued in connection with a securitization and represent a beneficial interest in a privately sponsored trust or other entity, the assets of which are mortgage loans or GNMA, FNMA, FHLMC or other mortgage-backed securities, including CMOs.

The Fund expects to invest a substantial amount (i.e., in excess of 10%), and from time to time a majority, of its total assets in TSOs.

A “Puerto Rico security” is any security that satisfies one or more of the following criteria: (i) securities of issuers that are organized under the laws of Puerto Rico or that maintain their principal place of business in Puerto Rico; (ii) securities that are traded principally in Puerto Rico; or (iii) securities of issuers that, during the issuer’s most recent fiscal year, derived at least 20% of their revenues or profits from goods produced or sold, investments made, or services performed in Puerto Rico or that have at least 20% of their assets in Puerto Rico.

The Fund’s investment objective may not be changed without the approval of the holders of a majority of the Fund’s outstanding voting securities (which for this purpose and under the 1940 Act, means the lesser of (i) 67% of the shares represented at a meeting at which more than 50% of the outstanding shares are represented or (ii) more than 50% of the outstanding shares).

Risk [Heading]

rr_RiskHeading

Principal Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

The following is a summary discussion of the principal risks of investing in the Fund. Risk is inherent in all investing. There can be no guarantee that the Fund will meet its investment objective or that the Fund’s performance will be positive for any period of time. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or by any bank or governmental agency. You may lose money by investing in the Fund.

Investment and Market Risk. The Fund’s investments may be adversely affected by the performance of U.S. and Puerto Rico investment securities markets, which, in turn, may be influenced by a number of factors, including, among other things, (i) the level of interest rates, (ii) the rate of inflation, (iii) political decisions, (iv) fiscal policy, and (v) current events in general. Because the Fund invests in investment securities, the Fund’s net asset value may fluctuate due to market conditions, and as a result you may experience a decline in the value of your investment in the Fund and you may lose money.

Coronavirus and Public Health Emergencies. As of the date of this prospectus, there is an outbreak of a novel and highly contagious form of coronavirus (COVID‑19) that has resulted in numerous disruptions in financial markets leaving general concern and uncertainty. As COVID‑19 continues to spread and mutate, the potential impacts, including a global, regional or other economic recession, are increasingly uncertain and difficult to assess. The impact of this coronavirus, and other epidemics and pandemics that may arise in the future, could affect the economies of many nations, individual companies and the market in general ways that cannot necessarily be foreseen at the present time.

Puerto Rico Concentration Risk. The Fund is exposed to certain risks resulting from the reduced geographic diversification of its portfolio. The Fund’s assets are invested primarily in securities of Puerto Rico issuers.

Because the Fund may invest a substantial portion of its assets in Puerto Rico bonds, the Fund’s net asset value and cash flow may fluctuate due to market conditions affecting these securities. Any future developments in this respect could result in additional interruptions in cash flow on debt payments, which may result in more price volatility, across Puerto Rico securities. There can be no assurance that any additional defaults by the Commonwealth and other Commonwealth instrumentalities will not have an additional adverse impact on the Fund’s net investment income. For a further description of developments in Puerto Rico, see Appendix A.

Consequently, the Fund’s performance may be more severely affected by economic, political, regulatory or other factors adversely affecting issuers in Puerto Rico than a fund that is not concentrated in Puerto Rico issuers.

Credit Risk. Credit risk is the risk that fixed income securities in the Fund’s portfolio will decline in price or that the issuer will fail to make principal or interest payments when due because the issuer of the security experiences a decline in its financial condition. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. To the extent that the Fund will typically invest a majority of its assets in securities issued by other investment companies for which, like the Fund, UBS Asset Managers of Puerto Rico acts as investment adviser or co‑investment adviser, and investing in similar types of assets, this risk may be magnified.

Interest Rate Risk. Interest rate risk is the risk that when interest rates rise, the value of the Fund’s investment will fall as a result. That is because the value of short-term instruments are generally expected to fall when short-term interest rates rise and to rise when short-term interest rates fall. Also, the Fund’s yield will tend to lag behind changes in prevailing short-term interest rates. This means that the Fund’s income will tend to rise more slowly than increases in short-term interest rates. Similarly, when short-term interest rates are falling, the Fund’s income will tend to fall more slowly.

Extension Risk. Extension risk is the risk that during periods of rising interest rates, the average life of certain types of securities may be extended because of the right of the issuer to defer payments or make slower than expected principal payments. This may lock in a below market interest rate, increase the security’s duration (the estimated period until the security is paid in full) and reduce the value of the security.

Prepayment Risk. Prepayment risk is the risk that during periods of declining interest rates, the issuer of a security may exercise its option to prepay principal earlier than scheduled in order to refinance at lower interest rates, forcing the Fund to reinvest in lower yielding securities. This tendency of issuers to refinance debt with high interest rates during periods of declining interest rates may reduce the positive effect of declining interest rates on the market value of the Fund’s securities.

General Municipal Securities Market Risk. The amount of public information available about the municipal securities in the Fund’s portfolio is generally less than that for corporate equities or bonds, and the Fund’s investment performance may therefore be more dependent on the analytical abilities of the Investment Adviser.

Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. Certain municipal securities, including private activity bonds, are not backed by the full faith, credit and taxing power of the issuer. Additionally, if events occur after the security is acquired that impact the security’s tax‑exempt status, the Fund and its shareholders could be subject to substantial tax liabilities.

Repurchase Agreements Risk. The Fund may also engage in repurchase agreements, which are transactions in which the Fund purchases a security from a counterparty and agrees to sell it back at a specified time and price in a specified currency. If a repurchase agreement counterparty defaults, the Fund may suffer time delays and incur costs or possible losses in connection with the disposition of the securities underlying the repurchase agreement.

Mortgage-Backed Securities Risk. Mortgage-backed securities have many of the risks of traditional debt securities but, in general, differ from investments in traditional debt securities in that, among other things, principal may be prepaid at any time due to prepayments by the obligors on the underlying obligations. Since a substantial portion of the assets of the Fund may be invested in mortgage-backed securities, the Fund may be subject to these risks and other risks related to such securities to a significant degree, which might cause the market value of the Fund’s investments to fluctuate more than otherwise would be the case. In addition, mortgage-backed or other securities issued or guaranteed by FNMA, FHLMC or a Federal Home Loan Bank are supported only by the credit of these entities and are not supported by the full faith and credit of the U.S.

CMO Risk. CMOs exhibit similar risks to those of mortgage-backed securities but also present certain special risks. CMO classes may be specially structured in a manner that provides a variety of investment characteristics, such as yield, effective maturity and interest rate sensitivity. As market conditions change, however, particularly during periods of rapid or unanticipated changes in interest rates, the ability of a CMO class to provide the anticipated investment characteristics and performance may be significantly reduced. These changes may result in volatility in the market value, and in some instances reduced liquidity, of the CMO class.

Asset-Backed Securities Risk. Asset-backed securities present risks similar to those of mortgage-backed securities. However, in the case of many asset-backed securities, the prepayment rates on the underlying assets have historically been less influenced by market interest rate fluctuations and therefore, have been more stable. The frequent absence of a government guarantee creates greater exposure to the credit risk on the underlying obligations and depending on the structure, the credit risk of the sponsor of such obligations.

Asset-backed securities entail certain risks not presented by mortgage-backed securities, including the risk that in certain states it may be difficult to perfect the liens securing the collateral backing certain asset-backed securities. In addition, certain asset-backed securities are based on loans that are unsecured, which means that there is no collateral to seize if the underlying borrower defaults.

Non‑Diversification Risk. As a non‑diversified fund, the Fund may invest a relatively high percentage of its assets in a small number of issuers. As a result, the value of the shares is, therefore, more susceptible to losses related to any single economic, political or regulatory occurrence than the value of shares of a more widely diversified fund.

Illiquid Investments Risk. An illiquid investment is any investment that the Fund reasonably expects cannot be sold or disposed of in current market conditions in seven calendar days or less without the sale or disposition significantly changing the market value of the investment. There presently are a limited number of participants in the market for certain Puerto Rico securities or other securities or assets that the Fund may own. That and other factors may cause certain securities to have periods of illiquidity.

Conflicts of Interest Risk. Conflicts of Interest Risk is the risk that the Investment Adviser and its affiliates, as well other investment companies advised or co‑advised by the Investment Adviser that issue TSOs purchased by the Fund, may have interests that compete with those of the Fund. Among other things, the Investment Adviser or its affiliates may engage in transactions directly with the Fund to the extent permitted by the 1940 Act. The Investment Adviser and its affiliates will also act in numerous other capacities in connection with the Fund and the Puerto Rico investment companies in which the Fund invests. These relationships also make the Fund very dependent upon the Investment Adviser and its affiliates. To the extent that the Fund invests in TSOs issued by investment companies affiliated with the Investment Adviser, such investments present certain conflict of interest, such as competing objectives with respect to the level of interest rates on the TSOs, and other related issues.

Risk Lose Money [Text]

rr_RiskLoseMoney

You may lose money by investing in the Fund.

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or by any bank or governmental agency.

Risk Nondiversified Status [Text]

rr_RiskNondiversifiedStatus

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance Information

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the ICE BofA US 3‑Month Treasury Bill Index (the “Benchmark Index”). To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. The table includes all applicable fees. If the Administrator had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance can be obtained by visiting www.ubs.com/prfunds or can be obtained by phone at (787) 250‑3600.

The Fund’s financial performance included in this Prospectus includes the Fund’s performance from a period when the Fund was not subject to the restrictions of a United States registered investment company that is subject to the requirements of the Investment Company Act of 1940, as amended.

The Benchmark Index measures the performance of a single issue of outstanding treasury bill which matures closest to, but not beyond, three months from the rebalancing date. The issue is purchased at the beginning of the month and held for a full month; at the end of the month that issue is sold and rolled into a newly selected issue.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

(787) 250‑3600

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.ubs.com/prfunds

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

As with all such investments, past performance (before and after taxes) is not an indication of future results.

Bar Chart [Heading]

rr_BarChartHeading

Total return

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the ten‑year period shown in the bar chart, the highest return for a quarter was 0.35% (quarter ended June 30, 2019) and the lowest return for a quarter was 0% (quarter ended December 31, 2021). The year‑to‑date return as of March 31, 2022 was 0%.

Performance Table Heading

rr_PerformanceTableHeading

For the periods ended 12/31/21 Average Annual Total Returns

Puerto Rico Short Term Investment Fund, Inc. | Class A

Distribution and/or Service (12b-1) Fees

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.50%

rr_DistributionAndService12b1FeesOverAssets

none

Administration

rr_Component1OtherExpensesOverAssets

0.05%

Miscellaneous

rr_Component2OtherExpensesOverAssets

0.16%

Other Expenses

rr_OtherExpensesOverAssets

0.21%

^[1]

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.71%

^[2]

Less Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

none

^[1]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.71%

^[2]

1 year

rr_ExpenseExampleYear01

$ 73

3 years

rr_ExpenseExampleYear03

227

^[3]

5 years

rr_ExpenseExampleYear05

395

^[3]

10 years

rr_ExpenseExampleYear10

$ 883

^[3]

2012

rr_AnnualReturn2012

0.04%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.01%

2017

rr_AnnualReturn2017

0.14%

2018

rr_AnnualReturn2018

0.74%

2019

rr_AnnualReturn2019

1.24%

2020

rr_AnnualReturn2020

0.12%

2021

rr_AnnualReturn2021

0.01%

Year to Date Return, Label

rr_YearToDateReturnLabel

year‑to‑date return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2022

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

none

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

highest return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2019

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.35%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

lowest return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2021

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

rr_AverageAnnualReturnYear01

0.01%

rr_AverageAnnualReturnYear05

0.45%

Puerto Rico Short Term Investment Fund, Inc. | Return After Taxes on Distributions | Class A

rr_AverageAnnualReturnYear10

0.23%

rr_RiskReturnAbstract

rr_AverageAnnualReturnYear01

0.01%

rr_AverageAnnualReturnYear05

0.26%

Puerto Rico Short Term Investment Fund, Inc. | Return After Taxes on Distributions and Sales of Shares | Class A

rr_AverageAnnualReturnYear10

0.14%

rr_RiskReturnAbstract

rr_AverageAnnualReturnYear01

0.01%

rr_AverageAnnualReturnYear05

0.26%

Puerto Rico Short Term Investment Fund, Inc. | ICE BofA US 3-Month Treasury Bill Index

rr_AverageAnnualReturnYear10

0.14%

rr_RiskReturnAbstract

rr_AverageAnnualReturnYear01

0.05%

rr_AverageAnnualReturnYear05

1.14%