v3.26.1
Fees and Expenses - Loomis Sayles Funds I - Classes Admin, Retail, Institutional, and N
Dec. 31, 2025
Loomis Sayles Income Fund  
Prospectus [Line Items]  
Expense Heading [Optional Text] FUND FEES & EXPENSES
Expense Narrative [Text Block]
The following table describes the fees and expenses that you may pay if you buy, hold, and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in this table.
The Fund does not impose a sales charge, a redemption fee or an exchange fee.
Operating Expenses Caption [Optional Text] Annual Fund Operating Expenses(expenses that you pay each year as a percentage of the value of your investment)
Annual Fund Operating Expenses [Table]
(expenses that you pay each year as a percentage of the value of your investment)
Institutional Class
Retail Class
Admin Class
Class N
Management fees
0.56%
0.56%
0.56%
0.56%
Distribution and/or service (12b-1) fees
0.00%
0.25%
0.25%
0.00%
Other expenses
0.15%
0.15%
0.39%1
0.07%
Total annual fund operating expenses
0.71%
0.96%
1.20%
0.63%
Fee waiver and/or expense reimbursement2
0.07%
0.07%
0.06%3
0.04%
Total annual fund operating expenses after fee waiver and/or expense reimbursement
0.64%
0.89%
1.14%
0.59%
[1],[2],[3]
Expense Example [Heading] Example
Expense Example Narrative [Text Block]
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 that 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 assuming that such waiver and/or reimbursement will only be in place through the date noted above and on the Total Annual Fund Operating Expenses for the remaining periods. The example does not take into account brokerage commissions and other fees to financial intermediaries that you may pay on your purchases and sales of shares of the Fund. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example, With Redemption [Table]
 
1 year
3 years
5 years
10 years
Institutional Class
$
65
$
220
$
388
$
876
Retail Class
$
91
$
299
$
524
$
1,172
Admin Class
$
116
$
375
$
654
$
1,449
Class N
$
60
$
198
$
347
$
782
Portfolio Turnover [Heading] Portfolio Turnover
Portfolio Turnover [Text Block]
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 104% of the average value of its portfolio.
Portfolio Turnover, Rate 104.00%
Loomis Sayles Investment Grade Fixed Income Fund  
Prospectus [Line Items]  
Expense Heading [Optional Text] FUND FEES & EXPENSES
Expense Narrative [Text Block]
The following table describes the fees and expenses that you may pay if you buy, hold, and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in this table.
The Fund does not impose a sales charge, a redemption fee or an exchange fee.
Operating Expenses Caption [Optional Text] Annual Fund Operating Expenses(expenses that you pay each year as a percentage of the value of your investment)
Annual Fund Operating Expenses [Table]
(expenses that you pay each year as a percentage of the value of your investment)
Institutional Class
Management fees
0.40%
Distribution and/or service (12b-1) fees
0.00%
Other expenses
0.19%
Total annual fund operating expenses
0.59%
Fee waiver and/or expense reimbursement1
0.06%
Total annual fund operating expenses after fee waiver and/or expense reimbursement
0.53%
[4]
Expense Example [Heading] Example
Expense Example Narrative [Text Block]
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 that 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 assuming that such waiver and/or reimbursement will only be in place through the date noted above and on the Total Annual Fund Operating Expenses for the remaining periods. The example does not take into account brokerage commissions and other fees to financial intermediaries that you may pay on your purchases and sales of shares of the Fund. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example, With Redemption [Table]
 
1 year
3 years
5 years
10 years
Institutional Class
$
54
$
183
$
323
$
732
Portfolio Turnover [Heading] Portfolio Turnover
Portfolio Turnover [Text Block]
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 76% of the average value of its portfolio.
Portfolio Turnover, Rate 76.00%
[1]
1 Other expenses include an administrative services fee of 0.24% for Admin Class shares.
[2]
2 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 0.64%, 0.89%, 1.14% and 0.59% of the Fund’s average daily net assets for Institutional Class shares, Retail Class shares, Admin Class shares and Class N shares, respectively, exclusive of acquired fund fees and expenses, brokerage expenses, interest expense, taxes, and organizational and extraordinary expenses, such as litigation and indemnification expenses. This undertaking is in effect through April 30, 2027 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 both (1) the class’ applicable expense limitation at the time such amounts were waived/reimbursed and (2) the class’ current applicable expense limitation. 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.
[3]
3 In order to ensure that the total annual fund operating expenses after fee waiver and/or expense reimbursement do not exceed the amounts disclosed in the table, the Adviser may voluntarily waive additional advisory fees and/or other expenses. This may result in Admin Class shareholders realizing a total annual fund operating expense after fee waiver and/or expense reimbursement lower than 1.14% of the Fund’s average daily net assets. This additional waiver may be terminated at any time.
[4]
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 0.53% of the Fund’s average daily net assets, 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 April 30, 2027 and may be terminated before then only with the consent of the Fund’s Board of Trustees. The Adviser will be permitted to recover management fees waived and/or expenses reimbursed to the extent that expenses in later periods fall below both (1) the expense limitation at the time such amounts were waived/reimbursed and (2) the Fund’s current applicable expense limitation. 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.