| Form N-CSR Cover | 12 Months Ended |
| Oct. 31, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | BlackRock Municipal Bond Fund, Inc. |
| Entity Central Index Key | 0000225635 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Oct. 31, 2024 |
| Shareholder Report | 12 Months Ended | |
| Oct. 31, 2024 USD ($) Holding | ||
| Shareholder Report [Line Items] | ||
| Document Type | N-CSR | |
| Amendment Flag | false | |
| Registrant Name | BlackRock Municipal Bond Fund, Inc. | |
| Entity Central Index Key | 0000225635 | |
| Entity Investment Company Type | N-1A | |
| Document Period End Date | Oct. 31, 2024 | |
| C000032428 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock High Yield Municipal Fund | |
| Class Name | Institutional Shares | |
| Trading Symbol | MAYHX | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about BlackRock High Yield Municipal Fund (the “Fund”) for the period of July 1, 2024 to October 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the period ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Institutional Shares $ (a) 0.58 % (b) (a) Expenses for a full reporting period would be higher than the amount shown. (b) Annualized. | |
| Expenses Paid, Amount | $ 20 | [1] |
| Expense Ratio, Percent | 0.58% | [2] |
| Factors Affecting Performance [Text Block] | How did the Fund perform during the period ? For the reporting period ended October 31, 2024, the Fund’s Institutional Shares returned 0.86%. For the same period, the Bloomberg Municipal Bond Index returned 1.21% and the Bloomberg Municipal High Yield Bond Index returned 1.64%. What contributed to performance? At the sector level, allocations to tax-backed states, education and healthcare issues made the largest contributions to absolute performance. In terms of credit tiers, non-rated and AA rated bonds were the leading contributors. Bonds with maturities of 25 years and above also helped results. What detracted from performance? At a time of positive returns for the broader market, few aspects of the Fund’s positioning hurt absolute performance. With that said, the Fund’s use of U.S. Treasury futures to manage interest-rate risk was a modest detractor. The Fund’s cash position was modestly elevated at the end of the period, largely to address liquidity needs and provide the flexibility to capitalize on opportunities as they arise. The cash position did not have a material impact on results. The views expressed reflect the opinions of BlackRock as of the date of this report and are subject to change based on changes in market, economic or other conditions. These views are not intended to be a forecast of future events and are no guarantee of future results. | |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Line Graph [Table Text Block] | Fund performance Cumulative performance: November 1, 2014 through October 31, 2024 Initial investment of $10,000 See “Average annual total returns” for additional information on fund performance. | |
| Average Annual Return [Table Text Block] | Average annual total returns 1 Year 5 Years 10 Years Institutional Shares 19.20 % 1.96 % 3.83 % Bloomberg Municipal Bond Index 9.70 1.05 2.30 Bloomberg Municipal High Yield Bond Index 17.47 2.71 4.40 High Yield Customized Reference Benchmark 15.14 2.22 N/A | |
| No Deduction of Taxes [Text Block] | Performance results do not reflect the deduction of taxes that a shareholder would pay on fund distributions or on the redemption or sale of fund shares. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com | |
| Net Assets | $ 1,585,113,158 | |
| Holdings Count | Holding | 698 | |
| Advisory Fees Paid, Amount | $ 2,481,961 | |
| Investment Company Portfolio Turnover | 15% | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 698 Net Investment Advisory Fees $ Portfolio Turnover Rate 15 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of October 31, 2024) Sector allocation Sector (a) Percent of Total (b) State 23.0 % Corporate 16.2 % Education 13.5 % County/City/Special District/School District 12.0 % Health 11.9 % Transportation 10.5 % Utilities 4.9 % Housing 4.2 % Tobacco 3.6 % Commercial Mortgage-Backed Securities 0.2 % Credit quality allocation Credit Rating (c) Percent of Total (b) AAA/Aaa 0.3 % AA/Aa 6.9 % A 9.0 % BBB/Baa 9.0 % BB/Ba 9.1 % B 2.2 % CCC/Caa 0.3 % N/R (d) 63.2 % (a) For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes. (b) Excludes short-term securities. (c) For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. (d) The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments. | |
| Credit Quality Explanation [Text Block] | The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments. | |
| Credit Ratings Selection [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Material Fund Change [Text Block] | Material Fund changes This is a summary of certain changes and planned changes to the Fund since June 30, 2024. For more complete information, you may review the Fund’s prospectus at blackrock.com/fundreports On September 13, 2024, the Fund’s Board approved the reorganization of the Fund into a newly created ETF to be named iShares High Yield Muni Active ETF. The reorganization is expected to close as of the close of trading on the New York Stock Exchange | |
| Summary of Change Legend [Text Block] | This is a summary of certain changes and planned changes to the Fund since June 30, 2024. For more complete information, you may review the Fund’s prospectus at blackrock.com/fundreports | |
| Updated Prospectus Phone Number | (800) 441-7762 | |
| Updated Prospectus Web Address | blackrock.com/fundreports | |
| C000032425 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock High Yield Municipal Fund | |
| Class Name | Investor A Shares | |
| Trading Symbol | MDYHX | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about BlackRock High Yield Municipal Fund (the “Fund”) for the period of July 1, 2024 to October 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the period ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Investor A Shares $ (a) 0.83 % (b) (a) Expenses for a full reporting period would be higher than the amount shown. (b) Annualized. | |
| Expenses Paid, Amount | $ 28 | [3] |
| Expense Ratio, Percent | 0.83% | [4] |
| Factors Affecting Performance [Text Block] | How did the Fund perform during the period ? For the reporting period ended October 31, 2024, the Fund’s Investor A Shares returned 0.78%. For the same period, the Bloomberg Municipal Bond Index returned 1.21% and the Bloomberg Municipal High Yield Bond Index returned 1.64%. What contributed to performance? At the sector level, allocations to tax-backed states, education and healthcare issues made the largest contributions to absolute performance. In terms of credit tiers, non-rated and AA rated bonds were the leading contributors. Bonds with maturities of 25 years and above also helped results. What detracted from performance? At a time of positive returns for the broader market, few aspects of the Fund’s positioning hurt absolute performance. With that said, the Fund’s use of U.S. Treasury futures to manage interest-rate risk was a modest detractor. The Fund’s cash position was modestly elevated at the end of the period, largely to address liquidity needs and provide the flexibility to capitalize on opportunities as they arise. The cash position did not have a material impact on results. The views expressed reflect the opinions of BlackRock as of the date of this report and are subject to change based on changes in market, economic or other conditions. These views are not intended to be a forecast of future events and are no guarantee of future results. | |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Line Graph [Table Text Block] | Fund performance Cumulative performance: November 1, 2014 through October 31, 2024 Initial investment of $10,000 See “Average annual total returns” for additional information on fund performance. | |
| Average Annual Return [Table Text Block] | Average annual total returns 1 Year 5 Years 10 Years Investor A Shares 18.95 % 1.73 % 3.57 % Investor A Shares (with sales charge) 13.89 0.85 3.12 Bloomberg Municipal Bond Index 9.70 1.05 2.30 Bloomberg Municipal High Yield Bond Index 17.47 2.71 4.40 High Yield Customized Reference Benchmark 15.14 2.22 N/A | |
| No Deduction of Taxes [Text Block] | Performance results do not reflect the deduction of taxes that a shareholder would pay on fund distributions or on the redemption or sale of fund shares. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com | |
| Net Assets | $ 1,585,113,158 | |
| Holdings Count | Holding | 698 | |
| Advisory Fees Paid, Amount | $ 2,481,961 | |
| Investment Company Portfolio Turnover | 15% | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 698 Net Investment Advisory Fees $ Portfolio Turnover Rate 15 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of October 31, 2024) Sector allocation Sector (a) Percent of Total (b) State 23.0 % Corporate 16.2 % Education 13.5 % County/City/Special District/School District 12.0 % Health 11.9 % Transportation 10.5 % Utilities 4.9 % Housing 4.2 % Tobacco 3.6 % Commercial Mortgage-Backed Securities 0.2 % Credit quality allocation Credit Rating (c) Percent of Total (b) AAA/Aaa 0.3 % AA/Aa 6.9 % A 9.0 % BBB/Baa 9.0 % BB/Ba 9.1 % B 2.2 % CCC/Caa 0.3 % N/R (d) 63.2 % (a) For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes. (b) Excludes short-term securities. (c) For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. (d) The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments. | |
| Credit Quality Explanation [Text Block] | The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments. | |
| Credit Ratings Selection [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Material Fund Change [Text Block] | Material Fund changes This is a summary of certain changes and planned changes to the Fund since June 30, 2024. For more complete information, you may review the Fund’s prospectus at blackrock.com/fundreports On September 13, 2024, the Fund’s Board approved the reorganization of the Fund into a newly created ETF to be named iShares High Yield Muni Active ETF. The reorganization is expected to close as of the close of trading on the New York Stock Exchange | |
| Summary of Change Legend [Text Block] | This is a summary of certain changes and planned changes to the Fund since June 30, 2024. For more complete information, you may review the Fund’s prospectus at blackrock.com/fundreports | |
| Updated Prospectus Phone Number | (800) 441-7762 | |
| Updated Prospectus Web Address | blackrock.com/fundreports | |
| C000032427 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock High Yield Municipal Fund | |
| Class Name | Investor C Shares | |
| Trading Symbol | MCYHX | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about BlackRock High Yield Municipal Fund (the “Fund”) for the period of July 1, 2024 to October 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the period ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Investor C Shares $ (a) 1.58 % (b) (a) Expenses for a full reporting period would be higher than the amount shown. (b) Annualized. | |
| Expenses Paid, Amount | $ 53 | [5] |
| Expense Ratio, Percent | 1.58% | [6] |
| Factors Affecting Performance [Text Block] | How did the Fund perform during the period ? For the reporting period ended October 31, 2024, the Fund’s Investor C Shares returned 0.42%. For the same period, the Bloomberg Municipal Bond Index returned 1.21% and the Bloomberg Municipal High Yield Bond Index returned 1.64%. What contributed to performance? At the sector level, allocations to tax-backed states, education and healthcare issues made the largest contributions to absolute performance. In terms of credit tiers, non-rated and AA rated bonds were the leading contributors. Bonds with maturities of 25 years and above also helped results. What detracted from performance? At a time of positive returns for the broader market, few aspects of the Fund’s positioning hurt absolute performance. With that said, the Fund’s use of U.S. Treasury futures to manage interest-rate risk was a modest detractor. The Fund’s cash position was modestly elevated at the end of the period, largely to address liquidity needs and provide the flexibility to capitalize on opportunities as they arise. The cash position did not have a material impact on results. The views expressed reflect the opinions of BlackRock as of the date of this report and are subject to change based on changes in market, economic or other conditions. These views are not intended to be a forecast of future events and are no guarantee of future results. | |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Line Graph [Table Text Block] | Fund performance Cumulative performance: November 1, 2014 through October 31, 2024 Initial investment of $10,000 See “Average annual total returns” for additional information on fund performance. | |
| Average Annual Return [Table Text Block] | Average annual total returns 1 Year 5 Years 10 Years Investor C Shares 17.88 % 0.95 % 2.94 % Investor C Shares (with sales charge) 16.88 0.95 2.94 Bloomberg Municipal Bond Index 9.70 1.05 2.30 Bloomberg Municipal High Yield Bond Index 17.47 2.71 4.40 High Yield Customized Reference Benchmark 15.14 2.22 N/A | |
| No Deduction of Taxes [Text Block] | Performance results do not reflect the deduction of taxes that a shareholder would pay on fund distributions or on the redemption or sale of fund shares. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com | |
| Net Assets | $ 1,585,113,158 | |
| Holdings Count | Holding | 698 | |
| Advisory Fees Paid, Amount | $ 2,481,961 | |
| Investment Company Portfolio Turnover | 15% | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 698 Net Investment Advisory Fees $ Portfolio Turnover Rate 15 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of October 31, 2024) Sector allocation Sector (a) Percent of Total (b) State 23.0 % Corporate 16.2 % Education 13.5 % County/City/Special District/School District 12.0 % Health 11.9 % Transportation 10.5 % Utilities 4.9 % Housing 4.2 % Tobacco 3.6 % Commercial Mortgage-Backed Securities 0.2 % Credit quality allocation Credit Rating (c) Percent of Total (b) AAA/Aaa 0.3 % AA/Aa 6.9 % A 9.0 % BBB/Baa 9.0 % BB/Ba 9.1 % B 2.2 % CCC/Caa 0.3 % N/R (d) 63.2 % (a) For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes. (b) Excludes short-term securities. (c) For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. (d) The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments. | |
| Credit Quality Explanation [Text Block] | The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments. | |
| Credit Ratings Selection [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Material Fund Change [Text Block] | Material Fund changes This is a summary of certain changes and planned changes to the Fund since June 30, 2024. For more complete information, you may review the Fund’s prospectus at blackrock.com/fundreports On September 13, 2024, the Fund’s Board approved the reorganization of the Fund into a newly created ETF to be named iShares High Yield Muni Active ETF. The reorganization is expected to close as of the close of trading on the New York Stock Exchange | |
| Summary of Change Legend [Text Block] | This is a summary of certain changes and planned changes to the Fund since June 30, 2024. For more complete information, you may review the Fund’s prospectus at blackrock.com/fundreports | |
| Updated Prospectus Phone Number | (800) 441-7762 | |
| Updated Prospectus Web Address | blackrock.com/fundreports | |
| C000199786 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock High Yield Municipal Fund | |
| Class Name | Class K Shares | |
| Trading Symbol | MKYHX | |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about BlackRock High Yield Municipal Fund (the “Fund”) for the period of July 1, 2024 to October 31, 2024. | |
| Shareholder Report Annual or Semi-Annual | annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the period ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Class K Shares $ (a) 0.53 % (b) (a) Expenses for a full reporting period would be higher than the amount shown. (b) Annualized. | |
| Expenses Paid, Amount | $ 18 | [7] |
| Expense Ratio, Percent | 0.53% | [8] |
| Factors Affecting Performance [Text Block] | How did the Fund perform during the period ? For the reporting period ended October 31, 2024, the Fund’s Class K Shares returned 0.88%. For the same period, the Bloomberg Municipal Bond Index returned 1.21% and the Bloomberg Municipal High Yield Bond Index returned 1.64%. What contributed to performance? At the sector level, allocations to tax-backed states, education and healthcare issues made the largest contributions to absolute performance. In terms of credit tiers, non-rated and AA rated bonds were the leading contributors. Bonds with maturities of 25 years and above also helped results. What detracted from performance? At a time of positive returns for the broader market, few aspects of the Fund’s positioning hurt absolute performance. With that said, the Fund’s use of U.S. Treasury futures to manage interest-rate risk was a modest detractor. The Fund’s cash position was modestly elevated at the end of the period, largely to address liquidity needs and provide the flexibility to capitalize on opportunities as they arise. The cash position did not have a material impact on results. The views expressed reflect the opinions of BlackRock as of the date of this report and are subject to change based on changes in market, economic or other conditions. These views are not intended to be a forecast of future events and are no guarantee of future results. | |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Line Graph [Table Text Block] | Fund performance Cumulative performance: November 1, 2014 through October 31, 2024 Initial investment of $10,000 See “Average annual total returns” for additional information on fund performance. | |
| Average Annual Return [Table Text Block] | Average annual total returns 1 Year 5 Years 10 Years Class K Shares 19.27 % 2.01 % 3.86 % Bloomberg Municipal Bond Index 9.70 1.05 2.30 Bloomberg Municipal High Yield Bond Index 17.47 2.71 4.40 High Yield Customized Reference Benchmark 15.14 2.22 N/A | |
| Performance Inception Date | Jan. 25, 2018 | |
| No Deduction of Taxes [Text Block] | Performance results do not reflect the deduction of taxes that a shareholder would pay on fund distributions or on the redemption or sale of fund shares. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com | |
| Net Assets | $ 1,585,113,158 | |
| Holdings Count | Holding | 698 | |
| Advisory Fees Paid, Amount | $ 2,481,961 | |
| Investment Company Portfolio Turnover | 15% | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 698 Net Investment Advisory Fees $ Portfolio Turnover Rate 15 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of October 31, 2024) Sector allocation Sector (a) Percent of Total (b) State 23.0 % Corporate 16.2 % Education 13.5 % County/City/Special District/School District 12.0 % Health 11.9 % Transportation 10.5 % Utilities 4.9 % Housing 4.2 % Tobacco 3.6 % Commercial Mortgage-Backed Securities 0.2 % Credit quality allocation Credit Rating (c) Percent of Total (b) AAA/Aaa 0.3 % AA/Aa 6.9 % A 9.0 % BBB/Baa 9.0 % BB/Ba 9.1 % B 2.2 % CCC/Caa 0.3 % N/R (d) 63.2 % (a) For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes. (b) Excludes short-term securities. (c) For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. (d) The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments. | |
| Credit Quality Explanation [Text Block] | The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments. | |
| Credit Ratings Selection [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Material Fund Change [Text Block] | Material Fund changes This is a summary of certain changes and planned changes to the Fund since June 30, 2024. For more complete information, you may review the Fund’s prospectus at blackrock.com/fundreports On September 13, 2024, the Fund’s Board approved the reorganization of the Fund into a newly created ETF to be named iShares High Yield Muni Active ETF. The reorganization is expected to close as of the close of trading on the New York Stock Exchange | |
| Summary of Change Legend [Text Block] | This is a summary of certain changes and planned changes to the Fund since June 30, 2024. For more complete information, you may review the Fund’s prospectus at blackrock.com/fundreports | |
| Updated Prospectus Phone Number | (800) 441-7762 | |
| Updated Prospectus Web Address | blackrock.com/fundreports | |
| [1]Expenses for a full reporting period would be higher than the amount shown.[2]Annualized.[3]Expenses for a full reporting period would be higher than the amount shown.[4]Annualized.[5]Expenses for a full reporting period would be higher than the amount shown.[6]Annualized.[7]Expenses for a full reporting period would be higher than the amount shown.[8]Annualized. | ||
| Shareholder Report, Line Graph (Details) - USD ($) | 12 Months Ended | 60 Months Ended | 120 Months Ended | ||||||||||
| Oct. 31, 2024 | Oct. 31, 2024 | Oct. 31, 2024 | Oct. 31, 2023 | Oct. 31, 2022 | Oct. 31, 2021 | Oct. 31, 2020 | Oct. 31, 2019 | Oct. 31, 2018 | Oct. 31, 2017 | Oct. 31, 2016 | Oct. 31, 2015 | Oct. 31, 2014 | |
| C000032428 [Member] | |||||||||||||
| Account Value [Line Items] | |||||||||||||
| Line Graph and Table Measure Name | Institutional Shares | ||||||||||||
| Account Value | $ 14,557 | $ 14,557 | $ 14,557 | $ 12,212 | $ 12,117 | $ 14,917 | $ 13,202 | $ 13,208 | $ 11,954 | $ 11,666 | $ 11,365 | $ 10,528 | $ 10,000 |
| Average Annual Return, Percent | 19.20% | 1.96% | 3.83% | ||||||||||
| C000032425 [Member] | |||||||||||||
| Account Value [Line Items] | |||||||||||||
| Line Graph and Table Measure Name | Investor A Shares (with sales charge) | ||||||||||||
| Average Annual Return, Percent | 13.89% | 0.85% | 3.12% | ||||||||||
| C000032425 [Member] | Without Sales Load [Member] | |||||||||||||
| Account Value [Line Items] | |||||||||||||
| Line Graph and Table Measure Name | Investor A Shares | ||||||||||||
| Account Value | $ 13,592 | $ 13,592 | $ 13,592 | 11,427 | 11,368 | 14,036 | 12,450 | 12,476 | 11,318 | 11,087 | 10,828 | 10,055 | 9,575 |
| Average Annual Return, Percent | 18.95% | 1.73% | 3.57% | ||||||||||
| C000032427 [Member] | |||||||||||||
| Account Value [Line Items] | |||||||||||||
| Line Graph and Table Measure Name | Investor C Shares (with sales charge) | ||||||||||||
| Average Annual Return, Percent | 16.88% | 0.95% | 2.94% | ||||||||||
| C000032427 [Member] | Without Sales Load [Member] | |||||||||||||
| Account Value [Line Items] | |||||||||||||
| Line Graph and Table Measure Name | Investor C Shares | ||||||||||||
| Account Value | $ 13,354 | $ 13,354 | $ 13,354 | 11,227 | 11,169 | 13,902 | 12,428 | 12,547 | 11,470 | 11,309 | 11,138 | 10,412 | 10,000 |
| Average Annual Return, Percent | 17.88% | 0.95% | 2.94% | ||||||||||
| C000199786 [Member] | |||||||||||||
| Account Value [Line Items] | |||||||||||||
| Line Graph and Table Measure Name | Class K Shares | ||||||||||||
| Account Value | $ 14,600 | $ 14,600 | $ 14,600 | 12,242 | 12,127 | 14,939 | 13,215 | 13,216 | 11,957 | 11,666 | 11,365 | 10,528 | 10,000 |
| Average Annual Return, Percent | 19.27% | 2.01% | 3.86% | ||||||||||
| Bloomberg Municipal Bond Index [Member] | |||||||||||||
| Account Value [Line Items] | |||||||||||||
| Line Graph and Table Measure Name | Bloomberg Municipal Bond Index | ||||||||||||
| Account Value | $ 12,548 | $ 12,548 | $ 12,548 | 11,439 | 11,144 | 12,661 | 12,336 | 11,908 | 10,883 | 10,939 | 10,704 | 10,287 | 10,000 |
| Average Annual Return, Percent | 9.70% | 1.05% | 2.30% | ||||||||||
| Bloomberg Municipal High Yield Bond Index [Member] | |||||||||||||
| Account Value [Line Items] | |||||||||||||
| Line Graph and Table Measure Name | Bloomberg Municipal High Yield Bond Index | ||||||||||||
| Account Value | $ 15,381 | $ 15,381 | $ 15,381 | $ 13,093 | $ 12,595 | $ 15,075 | $ 13,620 | $ 13,454 | $ 12,051 | $ 11,505 | $ 11,171 | $ 10,285 | $ 10,000 |
| Average Annual Return, Percent | 17.47% | 2.71% | 4.40% | ||||||||||
| High Yield Customized Reference Benchmark [Member] | |||||||||||||
| Account Value [Line Items] | |||||||||||||
| Line Graph and Table Measure Name | High Yield Customized Reference Benchmark | ||||||||||||
| Average Annual Return, Percent | 15.14% | 2.22% |
| Shareholder Report, Holdings (Details) | Oct. 31, 2024 | |
| C000032428 [Member] | N R [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 63.20% | [1],[2],[3] |
| C000032428 [Member] | Standard & Poor's, AAA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.30% | [1],[2] |
| C000032428 [Member] | Standard & Poor's, AA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 6.90% | [1],[2] |
| C000032428 [Member] | Standard & Poor's, A Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9% | [1],[2] |
| C000032428 [Member] | Standard & Poor's, BBB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9% | [1],[2] |
| C000032428 [Member] | Standard & Poor's, BB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.10% | [1],[2] |
| C000032428 [Member] | Standard & Poor's, B Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 2.20% | [1],[2] |
| C000032428 [Member] | Standard & Poor's, CCC Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.30% | [1],[2] |
| C000032428 [Member] | State [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 23% | [1],[4] |
| C000032428 [Member] | Corporate Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 16.20% | [1],[4] |
| C000032428 [Member] | Education [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 13.50% | [1],[4] |
| C000032428 [Member] | County City Special District School District [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 12% | [1],[4] |
| C000032428 [Member] | Healthcare Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 11.90% | [1],[4] |
| C000032428 [Member] | Transportation Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 10.50% | [1],[4] |
| C000032428 [Member] | Utilities Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 4.90% | [1],[4] |
| C000032428 [Member] | Housing [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 4.20% | [1],[4] |
| C000032428 [Member] | Tobacco [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 3.60% | [1],[4] |
| C000032428 [Member] | Commercial Mortgage-Backed Securities [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.20% | [1],[4] |
| C000032425 [Member] | N R [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 63.20% | [5],[6],[7] |
| C000032425 [Member] | Standard & Poor's, AAA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.30% | [5],[6] |
| C000032425 [Member] | Standard & Poor's, AA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 6.90% | [5],[6] |
| C000032425 [Member] | Standard & Poor's, A Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9% | [5],[6] |
| C000032425 [Member] | Standard & Poor's, BBB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9% | [5],[6] |
| C000032425 [Member] | Standard & Poor's, BB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.10% | [5],[6] |
| C000032425 [Member] | Standard & Poor's, B Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 2.20% | [5],[6] |
| C000032425 [Member] | Standard & Poor's, CCC Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.30% | [5],[6] |
| C000032425 [Member] | State [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 23% | [5],[8] |
| C000032425 [Member] | Corporate Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 16.20% | [5],[8] |
| C000032425 [Member] | Education [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 13.50% | [5],[8] |
| C000032425 [Member] | County City Special District School District [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 12% | [5],[8] |
| C000032425 [Member] | Healthcare Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 11.90% | [5],[8] |
| C000032425 [Member] | Transportation Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 10.50% | [5],[8] |
| C000032425 [Member] | Utilities Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 4.90% | [5],[8] |
| C000032425 [Member] | Housing [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 4.20% | [5],[8] |
| C000032425 [Member] | Tobacco [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 3.60% | [5],[8] |
| C000032425 [Member] | Commercial Mortgage-Backed Securities [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.20% | [5],[8] |
| C000032427 [Member] | N R [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 63.20% | [9],[10],[11] |
| C000032427 [Member] | Standard & Poor's, AAA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.30% | [9],[10] |
| C000032427 [Member] | Standard & Poor's, AA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 6.90% | [9],[10] |
| C000032427 [Member] | Standard & Poor's, A Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9% | [9],[10] |
| C000032427 [Member] | Standard & Poor's, BBB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9% | [9],[10] |
| C000032427 [Member] | Standard & Poor's, BB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.10% | [9],[10] |
| C000032427 [Member] | Standard & Poor's, B Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 2.20% | [9],[10] |
| C000032427 [Member] | Standard & Poor's, CCC Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.30% | [9],[10] |
| C000032427 [Member] | State [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 23% | [9],[12] |
| C000032427 [Member] | Corporate Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 16.20% | [9],[12] |
| C000032427 [Member] | Education [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 13.50% | [9],[12] |
| C000032427 [Member] | County City Special District School District [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 12% | [9],[12] |
| C000032427 [Member] | Healthcare Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 11.90% | [9],[12] |
| C000032427 [Member] | Transportation Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 10.50% | [9],[12] |
| C000032427 [Member] | Utilities Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 4.90% | [9],[12] |
| C000032427 [Member] | Housing [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 4.20% | [9],[12] |
| C000032427 [Member] | Tobacco [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 3.60% | [9],[12] |
| C000032427 [Member] | Commercial Mortgage-Backed Securities [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.20% | [9],[12] |
| C000199786 [Member] | N R [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 63.20% | [13],[14],[15] |
| C000199786 [Member] | Standard & Poor's, AAA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.30% | [13],[14] |
| C000199786 [Member] | Standard & Poor's, AA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 6.90% | [13],[14] |
| C000199786 [Member] | Standard & Poor's, A Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9% | [13],[14] |
| C000199786 [Member] | Standard & Poor's, BBB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9% | [13],[14] |
| C000199786 [Member] | Standard & Poor's, BB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.10% | [13],[14] |
| C000199786 [Member] | Standard & Poor's, B Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 2.20% | [13],[14] |
| C000199786 [Member] | Standard & Poor's, CCC Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.30% | [13],[14] |
| C000199786 [Member] | State [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 23% | [13],[16] |
| C000199786 [Member] | Corporate Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 16.20% | [13],[16] |
| C000199786 [Member] | Education [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 13.50% | [13],[16] |
| C000199786 [Member] | County City Special District School District [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 12% | [13],[16] |
| C000199786 [Member] | Healthcare Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 11.90% | [13],[16] |
| C000199786 [Member] | Transportation Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 10.50% | [13],[16] |
| C000199786 [Member] | Utilities Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 4.90% | [13],[16] |
| C000199786 [Member] | Housing [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 4.20% | [13],[16] |
| C000199786 [Member] | Tobacco [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 3.60% | [13],[16] |
| C000199786 [Member] | Commercial Mortgage-Backed Securities [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 0.20% | [13],[16] |
| [1]Excludes short-term securities.[2]For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change.[3]The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments.[4]For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes.[5]Excludes short-term securities.[6]For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change.[7]The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments.[8]For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes.[9]Excludes short-term securities.[10]For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change.[11]The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments.[12]For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes.[13]Excludes short-term securities.[14]For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change.[15]The investment adviser evaluates the credit quality of unrated investments based upon certain factors including, but not limited to, credit ratings for similar investments and financial analysis of sectors and individual investments. Using this approach, the investment adviser has deemed certain of these unrated securities as investment grade quality. As of October 31, 2024, the market value of unrated securities deemed by the investment adviser to be investment grade represents 1.0% of total investments.[16]For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes. | ||
