v3.25.3
S000017347 [Member] Average Annual Total Returns
 12 Months Ended 60 Months Ended 120 Months Ended 211 Months Ended
Jul. 31, 2025
Jul. 31, 2025
Jul. 31, 2025
Dec. 31, 2024
Bloomberg Municipal Bond Index (reflects no deduction for fees, expenses or taxes)        
Prospectus [Line Items]        
Average Annual Return, Percent [1] 1.05% 0.99% 2.25%  
Performance Inception Date [1]       May 31, 2007
Bloomberg 7 Year Municipal Bond Index (reflects no deduction for fees, expenses or taxes)        
Prospectus [Line Items]        
Average Annual Return, Percent [2] 0.51% 0.92% 2.00%  
Performance Inception Date [2]       May 31, 2007
Common Shares        
Prospectus [Line Items]        
Average Annual Return, Percent 2.69% 0.73% 2.71%  
Performance Inception Date       May 31, 2007
Common Shares | After Taxes on Distributions        
Prospectus [Line Items]        
Average Annual Return, Percent 2.69% 0.73% 2.71%  
Performance Inception Date       May 31, 2007
Common Shares | After Taxes on Distributions and Sales        
Prospectus [Line Items]        
Average Annual Return, Percent 3.33% 1.33% 2.89%  
Performance Inception Date       May 31, 2007
[1]
An index that measures the performance of the U.S. municipal bond market. It includes a diverse range of tax-exempt and taxable municipal bonds issued by state and local governments across the United States. The index is market-capitalization weighted and primarily consists of investment-grade bonds, providing investors with a snapshot of the overall performance of the municipal bond market.
[2]
An index designed to measure the performance of tax-exempt U.S. investment grade municipal bonds with remaining maturities between six and eight years.