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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName LOOMIS SAYLES FUNDS I
Prospectus Date rr_ProspectusDate Feb. 01, 2013
Loomis Sayles Intermediate Duration Bond Fund
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading loomis sayles intermediate duration bond fund
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The Fund's investment objective is above-average total return through a combination of current income and capital appreciation.
Expense [Heading] rr_ExpenseHeading FUND FEES & EXPENSES
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

The Fund does not impose a sales charge, a redemption fee or an exchange fee.
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 January 31, 2014
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 for you if your Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund's performance. During its most recently ended fiscal year, the Fund's portfolio turnover rate was 82% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 82.00%
Expense Example [Heading] rr_ExpenseExampleHeading

Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The 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 redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and the Fund's operating expenses remain the same, except that the example is based on the Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement for the first year and on the Total Annual Fund Operating Expenses for the remaining years. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading INVESTMENTS, RISKS AND PERFORMANCE

Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

Under normal circumstances, the Fund will invest at least 80% of its net assets (plus any borrowings made for investment purposes) in fixed-income securities (for example, bonds and other investments that Loomis Sayles believes have similar economic characteristics, such as notes, debentures and loans). It is anticipated that the Fund's weighted average duration will generally be between two and five years.

The Fund will purchase only investment-grade fixed-income securities, which are those securities that are rated as such at the time of purchase by at least one of the three major ratings agencies (Moody's Investors Service, Inc., Fitch Investor Services, Inc. or Standard & Poor's Ratings Group) or, if unrated, are determined by Loomis Sayles to be of comparable quality. The Fund may continue to hold up to 10% of its net assets in securities that are downgraded to a rating below investment-grade subsequent to their purchase if Loomis Sayles believes it is appropriate to do so.

In deciding which securities to buy and sell, Loomis Sayles may consider a number of factors related to the bond issue and the current bond market, for example, the stability and volatility of a country's bond markets, the financial strength of the issuer, current interest rates, current valuations, Loomis Sayles' expectations regarding general trends in interest rates and currency considerations. Loomis Sayles will also consider how purchasing or selling a bond would impact the overall portfolio's risk profile (for example, its sensitivity to currency risk, interest rate risk and sector-specific risk) and potential return (income and capital gains).

Three themes typically drive the Fund's investment approach. First, Loomis Sayles generally seeks fixed-income securities of issuers whose credit profiles it believes are improving. Loomis Sayles' credit research team provides deep fundamental and quantitative analysis as well as ratings on over 1,000 issuers worldwide. The broad coverage combined with the objective of identifying attractive investment opportunities makes this an important component of the investment approach. Second, the Fund may invest significantly in securities the prices of which Loomis Sayles believes are more sensitive to events related to the underlying issuer than to changes in general interest rates or overall market default rates. These securities may not have a direct correlation with changes in interest rates, thus helping to manage interest rate risk and to offer diversified sources for return. Third, Loomis Sayles analyzes different sectors of the economy and differences in the yields ("spreads") of various fixed-income securities (U.S. governments, investment-grade corporates, securitized assets, high-yield corporates, emerging markets, non-U.S. sovereigns and credits, convertibles, bank loans and municipals) in an effort to find securities that it believes may produce attractive returns for the Fund in comparison to their risk.

The Fund may invest any portion of its assets in U.S. dollar-denominated securities of Canadian issuers and up to 20% of its assets in other U.S. dollar-denominated foreign securities, including emerging market securities. The Fund may invest without limit in obligations of supranational entities ( e.g., the World Bank). The Fund may also invest in mortgage-related securities, including mortgage dollar rolls. The Fund may also engage in futures transactions, swaps (including credit default swaps, in which one party agrees to make periodic payments to a counterparty in exchange for the right to receive a payment in the event of a default of the underlying reference security).

The fixed-income securities in which the Fund may invest include, among other things, corporate bond and other debt securities (including junior and senior bonds), U.S. government securities, zero-coupon securities, mortgage-backed securities and other asset-backed securities, real estate investment trusts ("REITs"), securities issued pursuant to Rule 144A under the Securities Act of 1933 ("Rule 144A securities") and convertible securities.
Risk [Heading] rr_RiskHeading

Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

The principal risks of investing in the Fund are summarized below. The Fund does not represent a complete investment program. You may lose money by investing in the Fund.

Credit Risk is the risk that the issuer or guarantor of a fixed-income security in which the Fund invests, or the counterparty to a derivatives or other transaction will fail financially or otherwise be unwilling or unable to meet their obligations to the Fund.

Currency Risk is the risk that the value of the Fund's investments will fall as a result of changes in exchange rates.

Derivatives Risk is the risk that the value of the Fund's derivative investments such as futures transactions and swap transactions will fall, for example, because of changes in the value of the underlying reference instruments, pricing difficulties or lack of correlation with the underlying investments. There is also the risk that the Fund may be unable to terminate or sell a derivatives position at an advantageous time or price. Moreover, a number of broker-dealers and other financial institutions have recently experienced extreme financial difficulty, sometimes resulting in bankruptcy of the institution. There can be no assurance that the Fund's derivative counterparties will not experience similar financial difficulties, possibly resulting in losses to the Fund. This risk is greater for swaps and other over-the-counter traded derivatives. Investing in derivatives gives rise to other risks, such as leverage risk, liquidity risk, credit risk, counterparty risk, interest rate risk and market risk. The use of derivatives may cause the Fund to incur losses greater than those which would have occurred had derivatives not been used. The use of derivatives for other than hedging purposes may be considered a speculative activity, and involves greater risks than are involved in hedging.

Emerging Markets Risk is the risk that the Fund's investments may face greater foreign securities risk. Investing in companies traded in emerging securities markets, which may be smaller and have shorter operating histories than companies in developed markets, involves risks in addition to, and greater than, those generally associated with investing in developed foreign markets. The extent of economic development, political stability, market depth, infrastructure, capitalization and regulatory oversight in emerging market economies is generally less than in more developed markets.

Extension Risk is the risk that an unexpected rise in interest rates will extend the life of a mortgage- or asset-backed security beyond the expected prepayment time, typically reducing the security's value.

Foreign Securities Risk is the risk that the value of the Fund's foreign investments will fall as a result of foreign political, social, economic, environmental, credit, informational or currency changes or other issues relating to foreign investing generally. Foreign securities may be subject to higher volatility than U.S. securities, varying degrees of regulation and limited liquidity. The Fund's investments in foreign securities may be subject to foreign withholding taxes, which would decrease the yield on those securities.

Interest Rate Risk is the risk that the value of the Fund's investments will fall if interest rates rise. Interest rate risk generally is greater for funds that invest in fixed-income securities with relatively longer durations than for funds that invest in fixed-income securities with shorter durations. The value of zero-coupon securities and securities with longer maturities are generally more sensitive to fluctuations in interest rates than other fixed-income securities. In addition, an economic downturn or period of rising interest rates could adversely affect the market of these securities and reduce the Fund's ability to sell them.

Issuer Risk is the risk that the value of securities may decline due to a number of reasons relating to the issuer, such as management performance, financial leverage and reduced demand for the issuer's goods and services.

Large Investor Risk is the risk associated with ownership of shares of the Fund that may be concentrated in one or a few large investors. Such investors may redeem shares in large quantities or on a frequent basis. Redemptions by a large investor can affect the performance of the Fund, may increase realized capital gains, may accelerate the realization of taxable income to shareholders and may increase transaction costs. These transactions potentially limit the use of any capital loss carryforwards and certain other losses to offset future realized capital gains (if any). Such transactions may also increase the Fund's expenses.

Leverage Risk is the risk associated with securities or practices (e.g., borrowing and the use of certain derivatives) and investment in certain types of derivatives that multiply small index, market or asset price movements into larger changes in value. Use of derivative instruments may involve leverage. When a derivative is used as a hedge against an offsetting position that the Fund also holds, any loss generated by the derivative should be substantially offset by gains on the hedged instrument, and vice versa. To the extent that the Fund uses a derivative for purposes other than as a hedge, or if the Fund hedges imperfectly, the Fund is directly exposed to the risks of that derivative and any loss generated by the derivative will not be offset by a gain. Futures are derivatives and may be subject to this type of risk.

Liquidity Risk is the risk that the Fund may be unable to find a buyer for its investments when it seeks to sell them or to receive the price it expects. Securities acquired in a private placement, such as Rule 144A securities, generally are subject to strict restrictions on resale and there may be no liquid secondary market or ready purchaser for such securities. Therefore, the Fund may be unable to dispose of such securities when it desires to do so, or at the most advantageous time or price. Liquidity issues may also make it difficult to value the Fund's investments.

Management Risk is the risk that Loomis Sayles' investment techniques will be unsuccessful and cause the Fund to incur losses.

Market Risk is the risk that the market value of a security will move up and down, sometimes rapidly and unpredictably, based upon a change in an issuer's financial condition, as well as overall market and economic conditions.

Mortgage-Related and Asset-Backed Securities Risk is the risk that the securities may be prepaid and result in the reinvestment of the prepaid amounts in securities with lower yields than the prepaid obligations. Conversely, there is a risk that an unexpected rise in interest rates will extend the life of a mortgage-related or asset-backed security beyond the expected prepayment time, typically reducing the security's value. The Fund may also incur a loss when there is a prepayment of securities that were purchased at a premium. It also includes risks associated with investing in the mortgages underlying the mortgage-backed securities. The market for mortgage-backed securities (and other asset-backed securities) has experienced high volatility and a lack of liquidity. As a result, the value of many of these securities has significantly declined. The Fund's investments in other asset-backed securities are subject to risks similar to those associated with mortgage-related securities, as well as additional risks associated with the nature of the assets and the servicing of those assets.

REITs Risk is the risk that the value of the Fund's investments in REITs will fall as a result of changes in underlying real estate values, rising interest rates, limited diversification of holdings, higher costs and prepayment risk associated with related mortgages, as well as other risks particular to investments in real estate. Many REITs are highly leveraged, increasing their risk.

Risk Lose Money [Text] rr_RiskLoseMoney You may lose money by investing in the Fund.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading

Risk/Return Bar Chart and Table
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The following bar chart and table give an indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year and by showing how the Fund's average annual returns for the one-year, five-year, ten-year and life-of-fund periods compare to those of a broad measure of market performance. The Fund's past performance (before and after taxes) does not necessarily indicate how the Fund will perform in the future. Updated performance information is available online at www.loomissayles.com and/or by calling the Fund toll-free at 800-633-3330.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table give an indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year and by showing how the Fund's average annual returns for the one-year, five-year, ten-year and life-of-fund periods compare to those of a broad measure of market performance.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 800-633-3330
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.loomissayles.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund's past performance (before and after taxes) does not necessarily indicate how the Fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading

Total Returns for Institutional Class Shares
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

The Fund's best quarter was 7.49%, (second quarter of 2009), and the Fund's worst quarter was -2.93%, (third quarter of 2008).
Performance Table Heading rr_PerformanceTableHeading

Average Annual Total Returns for the periods ended December 31, 2012
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes Index performance reflects no deduction for fees, expenses or taxes.
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred After-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans, qualified plans, education savings accounts, such as 529 plans, or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown The after-tax returns are shown for the Institutional Class of the Fund. After-tax returns for the other classes of the Fund will vary.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock

Prior to the inception of Retail Class shares (May 28, 2010), performance is that of Institutional Class shares, restated to reflect the higher net expenses of Retail Class shares. The Fund revised its investment strategies on May 28, 2010; performance may have been different had the current investment strategies been in place for all periods shown.

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor's tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans, qualified plans, education savings accounts, such as 529 plans, or individual retirement accounts. The after-tax returns are shown for the Institutional Class of the Fund. After-tax returns for the other classes of the Fund will vary. Index performance reflects no deduction for fees, expenses or taxes.
Loomis Sayles Intermediate Duration Bond Fund | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.25%
Distribution and/or service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.26%
Total annual fund operating expenses rr_ExpensesOverAssets 0.51%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.11% [1]
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.40%
1 year rr_ExpenseExampleYear01 41
3 years rr_ExpenseExampleYear03 152
5 years rr_ExpenseExampleYear05 274
10 years rr_ExpenseExampleYear10 630
2003 rr_AnnualReturn2003 8.26%
2004 rr_AnnualReturn2004 3.35%
2005 rr_AnnualReturn2005 1.30%
2006 rr_AnnualReturn2006 4.44%
2007 rr_AnnualReturn2007 5.71%
2008 rr_AnnualReturn2008 (2.53%)
2009 rr_AnnualReturn2009 19.12%
2010 rr_AnnualReturn2010 7.66%
2011 rr_AnnualReturn2011 5.44%
2012 rr_AnnualReturn2012 5.62%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel best quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 7.49%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel worst quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (2.93%)
One Year rr_AverageAnnualReturnYear01 5.62%
Five Years rr_AverageAnnualReturnYear05 6.84%
Ten Years rr_AverageAnnualReturnYear10 5.71%
Life-of-Fund rr_AverageAnnualReturnSinceInception 5.79%
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 28, 1998
Loomis Sayles Intermediate Duration Bond Fund | Retail Class
  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.25%
Distribution and/or service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 0.34%
Total annual fund operating expenses rr_ExpensesOverAssets 0.84%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.19% [1]
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.65%
1 year rr_ExpenseExampleYear01 66
3 years rr_ExpenseExampleYear03 249
5 years rr_ExpenseExampleYear05 447
10 years rr_ExpenseExampleYear10 1,020
One Year rr_AverageAnnualReturnYear01 5.25%
Five Years rr_AverageAnnualReturnYear05 6.58%
Ten Years rr_AverageAnnualReturnYear10 5.41%
Life-of-Fund rr_AverageAnnualReturnSinceInception 5.45%
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 28, 1998
Loomis Sayles Intermediate Duration Bond Fund | Return After Taxes on Distributions | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 4.19%
Five Years rr_AverageAnnualReturnYear05 5.24%
Ten Years rr_AverageAnnualReturnYear10 4.01%
Life-of-Fund rr_AverageAnnualReturnSinceInception 3.80%
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 28, 1998
Loomis Sayles Intermediate Duration Bond Fund | Return After Taxes on Distributions and Sale of Fund Shares | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 3.75%
Five Years rr_AverageAnnualReturnYear05 4.93%
Ten Years rr_AverageAnnualReturnYear10 3.89%
Life-of-Fund rr_AverageAnnualReturnSinceInception 3.74%
Inception Date rr_AverageAnnualReturnInceptionDate Jan. 28, 1998
Loomis Sayles Intermediate Duration Bond Fund | Barclays U.S. Intermediate Government/Credit Bond Index
  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 3.89%
Five Years rr_AverageAnnualReturnYear05 5.18%
Ten Years rr_AverageAnnualReturnYear10 4.62%
Life-of-Fund rr_AverageAnnualReturnSinceInception 5.54%
[1] Loomis, Sayles & Company, L.P. ("Loomis Sayles" or the "Adviser") has given a binding contractual undertaking to the Fund to limit the amount of the Fund's total annual fund operating expenses to 0.40% and 0.65% of the Fund's average daily net assets for Institutional Class shares and Retail Class shares, respectively, exclusive of brokerage expenses, interest expense, taxes, acquired fund fees and expenses, organizational and extraordinary expenses, such as litigation and indemnification expenses. This undertaking is in effect through January 31, 2014 and may be terminated before then only with the consent of the Fund's Board of Trustees. The Adviser will be permitted to recover, on a class by class basis, management fees waived and/or expenses reimbursed to the extent that expenses in later periods fall below 0.40% and 0.65% of the Fund's average daily net assets for Institutional Class shares and Retail Class shares, respectively. The Fund will not be obligated to repay any such waived/reimbursed fees and expenses more than one year after the end of the fiscal year in which the fees or expenses were waived/reimbursed.