v3.26.3
Shareholder Report
 6 Months Ended
Aug. 31, 2026
USD ($)
$ / shares
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Franklin Municipal Securities Trust
Entity Central Index Key 0000881309
Entity Investment Company Type N-1A
Document Period End Date Aug. 31, 2026
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Class A  
Shareholder Report [Line Items]  
Fund Name Franklin California High Yield Municipal Fund
Class Name Class A
Trading Symbol FCQAX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about Franklin California High Yield Municipal Fund for the period March 1, 2026, to August 31, 2026.
Additional Information [Text Block] You can find additional information about the Fund at https://www.franklintempleton.com/regulatory-fund-documents. You can also request this information by contacting us at (800) DIAL BEN/342-5236.
Additional Information Phone Number (800) DIAL BEN/342-5236
Additional Information Website https://www.franklintempleton.com/regulatory-fund-documents
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST SIX MONTHS? (based on a hypothetical $10,000 investment)
Class Name
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment†
Class A
$40
0.80%
[1]
Expenses Paid, Amount $ 40
Expense Ratio, Percent 0.80%
Net Assets $ 3,301,186,543
Holdings Count | $ / shares 831
Investment Company Portfolio Turnover 3.43%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2026)
Total Net Assets
$3,301,186,543
Total Number of Portfolio Holdings
831
Portfolio Turnover Rate
3.43%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2026)
Portfolio Composition* (% of Total Investments)
image
 [2]
Updated Prospectus Web Address https://www.franklintempleton.com/regulatory-fund-documents
Class A1  
Shareholder Report [Line Items]  
Fund Name Franklin California High Yield Municipal Fund
Class Name Class A1
Trading Symbol FCAMX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about Franklin California High Yield Municipal Fund for the period March 1, 2026, to August 31, 2026.
Additional Information [Text Block] You can find additional information about the Fund at https://www.franklintempleton.com/regulatory-fund-documents. You can also request this information by contacting us at (800) DIAL BEN/342-5236.
Additional Information Phone Number (800) DIAL BEN/342-5236
Additional Information Website https://www.franklintempleton.com/regulatory-fund-documents
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST SIX MONTHS? (based on a hypothetical $10,000 investment)
Class Name
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment†
Class A1
$33
0.65%
[3]
Expenses Paid, Amount $ 33
Expense Ratio, Percent 0.65%
Net Assets $ 3,301,186,543
Holdings Count | $ / shares 831
Investment Company Portfolio Turnover 3.43%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2026)
Total Net Assets
$3,301,186,543
Total Number of Portfolio Holdings
831
Portfolio Turnover Rate
3.43%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2026)
Portfolio Composition* (% of Total Investments)
image
 [4]
Updated Prospectus Web Address https://www.franklintempleton.com/regulatory-fund-documents
Class C  
Shareholder Report [Line Items]  
Fund Name Franklin California High Yield Municipal Fund
Class Name Class C
Trading Symbol FCAHX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about Franklin California High Yield Municipal Fund for the period March 1, 2026, to August 31, 2026.
Additional Information [Text Block] You can find additional information about the Fund at https://www.franklintempleton.com/regulatory-fund-documents. You can also request this information by contacting us at (800) DIAL BEN/342-5236.
Additional Information Phone Number (800) DIAL BEN/342-5236
Additional Information Website https://www.franklintempleton.com/regulatory-fund-documents
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST SIX MONTHS? (based on a hypothetical $10,000 investment)
Class Name
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment†
Class C
$60
1.20%
[5]
Expenses Paid, Amount $ 60
Expense Ratio, Percent 1.20%
Net Assets $ 3,301,186,543
Holdings Count | $ / shares 831
Investment Company Portfolio Turnover 3.43%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2026)
Total Net Assets
$3,301,186,543
Total Number of Portfolio Holdings
831
Portfolio Turnover Rate
3.43%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2026)
Portfolio Composition* (% of Total Investments)
image
 [6]
Updated Prospectus Web Address https://www.franklintempleton.com/regulatory-fund-documents
Class R6  
Shareholder Report [Line Items]  
Fund Name Franklin California High Yield Municipal Fund
Class Name Class R6
Trading Symbol FCAQX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about Franklin California High Yield Municipal Fund for the period March 1, 2026, to August 31, 2026.
Additional Information [Text Block] You can find additional information about the Fund at https://www.franklintempleton.com/regulatory-fund-documents. You can also request this information by contacting us at (800) DIAL BEN/342-5236.
Additional Information Phone Number (800) DIAL BEN/342-5236
Additional Information Website https://www.franklintempleton.com/regulatory-fund-documents
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST SIX MONTHS? (based on a hypothetical $10,000 investment)
Class Name
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment†
Class R6
$25
0.50%
[7]
Expenses Paid, Amount $ 25
Expense Ratio, Percent 0.50%
Net Assets $ 3,301,186,543
Holdings Count | $ / shares 831
Investment Company Portfolio Turnover 3.43%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2026)
Total Net Assets
$3,301,186,543
Total Number of Portfolio Holdings
831
Portfolio Turnover Rate
3.43%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2026)
Portfolio Composition* (% of Total Investments)
image
 [8]
Updated Prospectus Web Address https://www.franklintempleton.com/regulatory-fund-documents
Advisor Class  
Shareholder Report [Line Items]  
Fund Name Franklin California High Yield Municipal Fund
Class Name Advisor Class
Trading Symbol FVCAX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about Franklin California High Yield Municipal Fund for the period March 1, 2026, to August 31, 2026.
Additional Information [Text Block] You can find additional information about the Fund at https://www.franklintempleton.com/regulatory-fund-documents. You can also request this information by contacting us at (800) DIAL BEN/342-5236.
Additional Information Phone Number (800) DIAL BEN/342-5236
Additional Information Website https://www.franklintempleton.com/regulatory-fund-documents
Expenses [Text Block]
WHAT WERE THE FUND COSTS FOR THE LAST SIX MONTHS? (based on a hypothetical $10,000 investment)
Class Name
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment†
Advisor Class
$28
0.55%
[9]
Expenses Paid, Amount $ 28
Expense Ratio, Percent 0.55%
Net Assets $ 3,301,186,543
Holdings Count | $ / shares 831
Investment Company Portfolio Turnover 3.43%
Additional Fund Statistics [Text Block]
KEY FUND STATISTICS (as of August 31, 2026)
Total Net Assets
$3,301,186,543
Total Number of Portfolio Holdings
831
Portfolio Turnover Rate
3.43%
Holdings [Text Block]
WHAT DID THE FUND INVEST IN? (as of August 31, 2026)
Portfolio Composition* (% of Total Investments)
image
 [10]
Updated Prospectus Web Address https://www.franklintempleton.com/regulatory-fund-documents
[1]
† Annualized.
[2]
* Does not include derivatives, except purchased options, if any.  
[3]
† Annualized.
[4]
* Does not include derivatives, except purchased options, if any.  
[5]
† Annualized.
[6]
* Does not include derivatives, except purchased options, if any.  
[7]
† Annualized.
[8]
* Does not include derivatives, except purchased options, if any.  
[9]
† Annualized.
[10]
* Does not include derivatives, except purchased options, if any.