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Shareholder Report
 12 Months Ended 68 Months Ended 92 Months Ended 140 Months Ended 172 Months Ended 297 Months Ended 321 Months Ended 333 Months Ended
Sep. 30, 2024
USD ($)
Holding
Sep. 30, 2024
USD ($)
Holding
Sep. 30, 2024
USD ($)
Holding
Sep. 30, 2024
USD ($)
Holding
Sep. 30, 2024
USD ($)
Holding
Sep. 30, 2024
USD ($)
Holding
Sep. 30, 2024
USD ($)
Holding
Sep. 30, 2024
USD ($)
Holding
Shareholder Report [Line Items]                
Document Type N-CSR              
Amendment Flag false              
Registrant Name LOOMIS SAYLES FUNDS I              
Entity Central Index Key 0000917469              
Entity Investment Company Type N-1A              
Document Period End Date Sep. 30, 2024              
C000018126                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Fixed Income Fund              
Class Name Institutional Class              
Trading Symbol LSFIX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Fixed Income Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 66              
Expense Ratio, Percent 0.61%              
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 459,951,656 $ 459,951,656 $ 459,951,656 $ 459,951,656 $ 459,951,656 $ 459,951,656 $ 459,951,656 $ 459,951,656
Holdings Count | Holding 665 665 665 665 665 665 665 665
Advisory Fees Paid, Amount $ 2,190,546              
InvestmentCompanyPortfolioTurnover 63.00%              
C000125494                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Global Bond Fund              
Class Name Class N              
Trading Symbol LSGNX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Global Bond Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 68              
Expense Ratio, Percent 0.64%              
Performance Inception Date       Feb. 01, 2013        
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274
Holdings Count | Holding 234 234 234 234 234 234 234 234
Advisory Fees Paid, Amount $ 1,793,932              
InvestmentCompanyPortfolioTurnover 63.00%              
C000018127                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Global Bond Fund              
Class Name Institutional Class              
Trading Symbol LSGBX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Global Bond Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 73              
Expense Ratio, Percent 0.69%              
Performance Inception Date               Dec. 30, 2096
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274
Holdings Count | Holding 234 234 234 234 234 234 234 234
Advisory Fees Paid, Amount $ 1,793,932              
InvestmentCompanyPortfolioTurnover 63.00%              
C000018128                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Global Bond Fund              
Class Name Retail Class              
Trading Symbol LSGLX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Global Bond Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 100              
Expense Ratio, Percent 0.94%              
Performance Inception Date               Dec. 30, 2096
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274 $ 370,778,274
Holdings Count | Holding 234 234 234 234 234 234 234 234
Advisory Fees Paid, Amount $ 1,793,932              
InvestmentCompanyPortfolioTurnover 63.00%              
C000018129                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles High Income Opportunities Fund              
Class Name Institutional Class              
Trading Symbol LSIOX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles High Income Opportunities Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at loomissayles.com/website/intermediary-advisory/prospectuses-and-fund-reports. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 0              
Expense Ratio, Percent 0.00%              
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 261,255,748 $ 261,255,748 $ 261,255,748 $ 261,255,748 $ 261,255,748 $ 261,255,748 $ 261,255,748 $ 261,255,748
Holdings Count | Holding 480 480 480 480 480 480 480 480
Advisory Fees Paid, Amount $ 0              
InvestmentCompanyPortfolioTurnover 88.00%              
C000182246                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Inflation Protected Securities Fund              
Class Name Class N              
Trading Symbol LIPNX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Inflation Protected Securities Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 37              
Expense Ratio, Percent 0.35%              
Performance Inception Date     Feb. 01, 2017          
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501
Holdings Count | Holding 43 43 43 43 43 43 43 43
Advisory Fees Paid, Amount $ 19,399              
InvestmentCompanyPortfolioTurnover 40.00%              
C000018130                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Inflation Protected Securities Fund              
Class Name Institutional Class              
Trading Symbol LSGSX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Inflation Protected Securities Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 42              
Expense Ratio, Percent 0.40%              
Performance Inception Date         May 27, 2010      
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501
Holdings Count | Holding 43 43 43 43 43 43 43 43
Advisory Fees Paid, Amount $ 19,399              
InvestmentCompanyPortfolioTurnover 40.00%              
C000089752                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Inflation Protected Securities Fund              
Class Name Retail Class              
Trading Symbol LIPRX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Inflation Protected Securities Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 68              
Expense Ratio, Percent 0.65%              
Performance Inception Date         May 27, 2010      
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501 $ 108,091,501
Holdings Count | Holding 43 43 43 43 43 43 43 43
Advisory Fees Paid, Amount $ 19,399              
InvestmentCompanyPortfolioTurnover 40.00%              
C000018131                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Institutional High Income Fund              
Class Name Institutional Class              
Trading Symbol LSHIX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Institutional High Income Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 78              
Expense Ratio, Percent 0.72%              
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 333,857,326 $ 333,857,326 $ 333,857,326 $ 333,857,326 $ 333,857,326 $ 333,857,326 $ 333,857,326 $ 333,857,326
Holdings Count | Holding 451 451 451 451 451 451 451 451
Advisory Fees Paid, Amount $ 1,887,370              
InvestmentCompanyPortfolioTurnover 90.00%              
C000089753                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Intermediate Duration Bond Fund              
Class Name Class A              
Trading Symbol LSDRX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Intermediate Duration Bond Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information (including tax information) about the Fund at im.natixis.com/funddocuments. You can also request the information by contacting us at (800) 225-5478 or by contacting your financial intermediary directly.              
Additional Information Phone Number (800) 225-5478              
Additional Information Website <span style="box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 12px; font-weight: 400; grid-area: auto; line-height: 18px; margin: 0px; overflow: visible; text-align: left;">im.natixis.com/funddocuments</span>              
Expenses Paid, Amount $ 68              
Expense Ratio, Percent 0.65%              
Performance Inception Date         May 27, 2010      
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303
Holdings Count | Holding 468 468 468 468 468 468 468 468
Advisory Fees Paid, Amount $ 521,187              
InvestmentCompanyPortfolioTurnover 132.00%              
C000173092                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Intermediate Duration Bond Fund              
Class Name Class C              
Trading Symbol LSCDX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Intermediate Duration Bond Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information (including tax information) about the Fund at im.natixis.com/funddocuments. You can also request the information by contacting us at (800) 225-5478 or by contacting your financial intermediary directly.              
Additional Information Phone Number (800) 225-5478              
Additional Information Website <span style="box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 12px; font-weight: 400; grid-area: auto; line-height: 18px; margin: 0px; overflow: visible; text-align: left;">im.natixis.com/funddocuments</span>              
Expenses Paid, Amount $ 146              
Expense Ratio, Percent 1.40%              
Performance Inception Date           Jan. 01, 2000    
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303
Holdings Count | Holding 468 468 468 468 468 468 468 468
Advisory Fees Paid, Amount $ 521,187              
InvestmentCompanyPortfolioTurnover 132.00%              
C000211126                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Intermediate Duration Bond Fund              
Class Name Class N              
Trading Symbol LSDNX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Intermediate Duration Bond Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information (including tax information) about the Fund at im.natixis.com/funddocuments. You can also request the information by contacting us at (800) 225-5478 or by contacting your financial intermediary directly.              
Additional Information Phone Number (800) 225-5478              
Additional Information Website <span style="box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 12px; font-weight: 400; grid-area: auto; line-height: 18px; margin: 0px; overflow: visible; text-align: left;">im.natixis.com/funddocuments</span>              
Expenses Paid, Amount $ 37              
Expense Ratio, Percent 0.35%              
Performance Inception Date   Feb. 01, 2019            
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303
Holdings Count | Holding 468 468 468 468 468 468 468 468
Advisory Fees Paid, Amount $ 521,187              
InvestmentCompanyPortfolioTurnover 132.00%              
C000018132                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Intermediate Duration Bond Fund              
Class Name Class Y              
Trading Symbol LSDIX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Intermediate Duration Bond Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information (including tax information) about the Fund at im.natixis.com/funddocuments. You can also request the information by contacting us at (800) 225-5478 or by contacting your financial intermediary directly.              
Additional Information Phone Number (800) 225-5478              
Additional Information Website <span style="box-sizing: border-box; color: rgb(0, 0, 0); display: inline; flex-wrap: nowrap; font-size: 12px; font-weight: 400; grid-area: auto; line-height: 18px; margin: 0px; overflow: visible; text-align: left;">im.natixis.com/funddocuments</span>              
Expenses Paid, Amount $ 42              
Expense Ratio, Percent 0.40%              
Performance Inception Date         May 27, 2010      
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303 $ 265,345,303
Holdings Count | Holding 468 468 468 468 468 468 468 468
Advisory Fees Paid, Amount $ 521,187              
InvestmentCompanyPortfolioTurnover 132.00%              
C000018125                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Securitized Asset Fund              
Class Name Institutional Class              
Trading Symbol LSSAX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Securitized Asset Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at loomissayles.com/website/intermediary-advisory/prospectuses-and-fund-reports. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 0              
Expense Ratio, Percent 0.00%              
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 1,492,251,789 $ 1,492,251,789 $ 1,492,251,789 $ 1,492,251,789 $ 1,492,251,789 $ 1,492,251,789 $ 1,492,251,789 $ 1,492,251,789
Holdings Count | Holding 623 623 623 623 623 623 623 623
Advisory Fees Paid, Amount $ 0              
InvestmentCompanyPortfolioTurnover 90.00%              
C000018136                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Small Cap Value Fund              
Class Name Admin Class              
Trading Symbol LSVAX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Small Cap Value Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 155              
Expense Ratio, Percent 1.40%              
Performance Inception Date             Dec. 31, 2097  
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104
Holdings Count | Holding 116 116 116 116 116 116 116 116
Advisory Fees Paid, Amount $ 2,614,608              
InvestmentCompanyPortfolioTurnover 25.00%              
C000125495                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Small Cap Value Fund              
Class Name Class N              
Trading Symbol LSCNX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Small Cap Value Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 95              
Expense Ratio, Percent 0.85%              
Performance Inception Date       Feb. 01, 2013        
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104
Holdings Count | Holding 116 116 116 116 116 116 116 116
Advisory Fees Paid, Amount $ 2,614,608              
InvestmentCompanyPortfolioTurnover 25.00%              
C000018134                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Small Cap Value Fund              
Class Name Institutional Class              
Trading Symbol LSSCX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Small Cap Value Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 100              
Expense Ratio, Percent 0.90%              
Performance Inception Date               Dec. 30, 2096
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104
Holdings Count | Holding 116 116 116 116 116 116 116 116
Advisory Fees Paid, Amount $ 2,614,608              
InvestmentCompanyPortfolioTurnover 25.00%              
C000018135                
Shareholder Report [Line Items]                
Fund Name Loomis Sayles Small Cap Value Fund              
Class Name Retail Class              
Trading Symbol LSCRX              
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains important information about Loomis Sayles Small Cap Value Fund for the period of October 1, 2023 to September 30, 2024.              
Shareholder Report Annual or Semi-Annual Annual Shareholder Report              
Additional Information [Text Block] You can find additional information about the Fund at im.natixis.com/funddocuments. You can also request this information by contacting us at (800) 633-3330 or by contacting your financial intermediary directly.              
Expenses Paid, Amount $ 128              
Expense Ratio, Percent 1.15%              
Performance Inception Date               Dec. 30, 2096
Previous Investment Adviser [Text Block]

 

              
AssetsNet $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104 $ 392,961,104
Holdings Count | Holding 116 116 116 116 116 116 116 116
Advisory Fees Paid, Amount $ 2,614,608              
InvestmentCompanyPortfolioTurnover 25.00%