v3.26.1
Shareholder Report
 6 Months Ended
Apr. 30, 2026
USD ($)
Holding
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name BNY MELLON INDEX FUNDS, INC.
Entity Central Index Key 0000857114
Entity Investment Company Type N-1A
Document Period End Date Apr. 30, 2026
C000000294 [Member]  
Shareholder Report [Line Items]  
Fund Name BNY Mellon International Stock Index Fund
Class Name Investor Shares
Trading Symbol DIISX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about BNY Mellon International Stock Index Fund (the “Fund”) for the period of November 1, 2025 to April 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at bny.com/investments/literaturecenter. You can also request this information by calling 1-800-373-9387 (inside the U.S. only) or by sending an e-mail request to info@bny.com.
Additional Information Phone Number 1-800-373-9387
Additional Information Email info@bny.com
Additional Information Website bny.com/investments/literaturecenter
Expenses [Text Block]
What were the Fund’s costs for the last six months ?
(based on a hypothetical $10,000 investment)
Share Class Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment
Investor Shares* $31 0.60%**
*
During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
**
Annualized.
Expenses Paid, Amount $ 31 [1]
Expense Ratio, Percent 0.60% [1],[2]
Net Assets $ 327,000,000
Holdings Count | Holding 693
Investment Company Portfolio Turnover 1.35%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (AS OF 4/30/26 )

Fund Size (Millions)
Number of Holdings
Portfolio Turnover
$327 693 1.35%
Holdings [Text Block]
Portfolio Holdings (as of 4/30/26 )
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
Country Allocation (Based on Net Assets)
Graphical Representation - Allocation 1 Chart
Largest Holdings [Text Block]
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
C000172467 [Member]  
Shareholder Report [Line Items]  
Fund Name BNY Mellon International Stock Index Fund
Class Name Class I
Trading Symbol DINIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about BNY Mellon International Stock Index Fund (the “Fund”) for the period of November 1, 2025 to April 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at bny.com/investments/literaturecenter. You can also request this information by calling 1-800-373-9387 (inside the U.S. only) or by sending an e-mail request to info@bny.com.
Additional Information Phone Number 1-800-373-9387
Additional Information Email info@bny.com
Additional Information Website bny.com/investments/literaturecenter
Expenses [Text Block]
What were the Fund’s costs for the last six months ?
(based on a hypothetical $10,000 investment)
Share Class Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment
Class I* $18 0.35%**
*
During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
**
Annualized.
Expenses Paid, Amount $ 18 [3]
Expense Ratio, Percent 0.35% [3],[4]
Net Assets $ 327,000,000
Holdings Count | Holding 693
Investment Company Portfolio Turnover 1.35%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (AS OF 4/30/26 )

Fund Size (Millions)
Number of Holdings
Portfolio Turnover
$327 693 1.35%
Holdings [Text Block]
Portfolio Holdings (as of 4/30/26 )
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
Country Allocation (Based on Net Assets)
Graphical Representation - Allocation 1 Chart
Largest Holdings [Text Block]
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
C000000295 [Member]  
Shareholder Report [Line Items]  
Fund Name BNY Mellon S&P 500 Index Fund
Class Name BNY Mellon S&P 500 Index Fund
Trading Symbol PEOPX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about BNY Mellon S&P 500 Index Fund (the “Fund”) for the period of November 1, 2025 to April 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at bny.com/investments/literaturecenter. You can also request this information by calling 1-800-373-9387 (inside the U.S. only) or by sending an e-mail request to info@bny.com.
Material Fund Change Notice [Text Block]
This report describes changes to the Fund that occurred during the reporting period.
Additional Information Phone Number 1-800-373-9387
Additional Information Email info@bny.com
Additional Information Website bny.com/investments/literaturecenter
Expenses [Text Block]
What were the Fund’s costs for the last six months ?
(based on a hypothetical $10,000 investment)
Fund Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment
BNY Mellon S&P 500 Index Fund* $26 0.50%**
*
During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
**
Annualized.
Expenses Paid, Amount $ 26 [5]
Expense Ratio, Percent 0.50% [5],[6]
Material Change Date Nov. 14, 2025
Net Assets $ 2,696,000,000
Holdings Count | Holding 506
Investment Company Portfolio Turnover 3.51%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (AS OF 4/30/26 )

Fund Size (Millions)
Number of Holdings
Portfolio Turnover
$2,696 506 3.51%
Holdings [Text Block]
Portfolio Holdings (as of 4/30/26 )
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
Sector Allocation (Based on Net Assets)
Graphical Representation - Allocation 1 Chart
Largest Holdings [Text Block]
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
Material Fund Change [Text Block]
How has the Fund changed?
  • Effective November 14, 2025, the Fund may operate as a non-diversified fund, as defined under the Investment Company Act of 1940, as amended, to the approximate extent the S&P 500® Index is non-diversified, without shareholder approval. The Fund may therefore operate as non-diversified solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the index.
This is a summary of certain changes to the Fund since November 1, 2025 . For more complete information, you may review the Fund’s current prospectus dated February 27, 2026 at bny.com/investments/literaturecenter or upon request at 1-800-373-9387.
Material Fund Change Risks Change [Text Block] Effective November 14, 2025, the Fund may operate as a non-diversified fund, as defined under the Investment Company Act of 1940, as amended, to the approximate extent the S&P 500® Index is non-diversified, without shareholder approval. The Fund may therefore operate as non-diversified solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the index.
Summary of Change Legend [Text Block]
This is a summary of certain changes to the Fund since November 1, 2025 . For more complete information, you may review the Fund’s current prospectus dated February 27, 2026 at bny.com/investments/literaturecenter or upon request at 1-800-373-9387.
Updated Prospectus Phone Number 1-800-373-9387
Updated Prospectus Web Address bny.com/investments/literaturecenter
C000172468 [Member]  
Shareholder Report [Line Items]  
Fund Name BNY Mellon Smallcap Stock Index Fund
Class Name Class I
Trading Symbol DISIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about BNY Mellon Smallcap Stock Index Fund (the “Fund”) for the period of November 1, 2025 to April 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at bny.com/investments/literaturecenter. You can also request this information by calling 1-800-373-9387 (inside the U.S. only) or by sending an e-mail request to info@bny.com.
Additional Information Phone Number 1-800-373-9387
Additional Information Email info@bny.com
Additional Information Website bny.com/investments/literaturecenter
Expenses [Text Block]
What were the Fund’s costs for the last six months ?
(based on a hypothetical $10,000 investment)
Share Class Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment
Class I* $13 0.25%**
*
During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
**
Annualized.
Expenses Paid, Amount $ 13 [7]
Expense Ratio, Percent 0.25% [7],[8]
Net Assets $ 845,000,000
Holdings Count | Holding 609
Investment Company Portfolio Turnover 28.66%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (AS OF 4/30/26 )

Fund Size (Millions)
Number of Holdings
Portfolio Turnover
$845 609 28.66%
Holdings [Text Block]
Portfolio Holdings (as of 4/30/26 )
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
Sector Allocation (Based on Net Assets)
Graphical Representation - Allocation 1 Chart
Largest Holdings [Text Block]
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
C000000296 [Member]  
Shareholder Report [Line Items]  
Fund Name BNY Mellon Smallcap Stock Index Fund
Class Name Investor Shares
Trading Symbol DISSX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about BNY Mellon Smallcap Stock Index Fund (the “Fund”) for the period of November 1, 2025 to April 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at bny.com/investments/literaturecenter. You can also request this information by calling 1-800-373-9387 (inside the U.S. only) or by sending an e-mail request to info@bny.com.
Additional Information Phone Number 1-800-373-9387
Additional Information Email info@bny.com
Additional Information Website bny.com/investments/literaturecenter
Expenses [Text Block]
What were the Fund’s costs for the last six months ?
(based on a hypothetical $10,000 investment)
Share Class Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment
Investor Shares* $27 0.50%**
*
During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
**
Annualized.
Expenses Paid, Amount $ 27 [9]
Expense Ratio, Percent 0.50% [9],[10]
Net Assets $ 845,000,000
Holdings Count | Holding 609
Investment Company Portfolio Turnover 28.66%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (AS OF 4/30/26 )

Fund Size (Millions)
Number of Holdings
Portfolio Turnover
$845 609 28.66%
Holdings [Text Block]
Portfolio Holdings (as of 4/30/26 )
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
Sector Allocation (Based on Net Assets)
Graphical Representation - Allocation 1 Chart
Largest Holdings [Text Block]
Top Ten Holdings (Based on Net Assets) *
Graphical Representation - Top N Holdings Chart
* Excludes money market funds or other short-term securities held for the investment of cash and cash collateral for securities loaned, if any.
[1]
During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
[2]
Annualized.
[3]
During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
[4]
Annualized.
[5] During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
[6] Annualized.
[7]
During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
[8]
Annualized.
[9] During the period, fees were waived and/or expenses reimbursed pursuant to an agreement with the Fund’s investment adviser, BNY Mellon Investment Adviser, Inc.
[10] Annualized.