v3.26.3
Average Annual Total Returns - AMERICAN CENTURY MUNICIPAL TRUST
 12 Months Ended 60 Months Ended 105 Months Ended 120 Months Ended
Oct. 01, 2026
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
S&P National AMT-Free Municipal Bond Index (reflects no deduction for fees, expenses or taxes)          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] S&P National AMT-Free Municipal Bond Index(reflects no deduction for fees, expenses or taxes)        
Average Annual Return, Percent   3.77% 0.84%   2.28%
S&P Municipal Bond 50% Investment Grade/50% High Yield Index (reflects no deduction for fees, expenses or taxes)          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] S&P Municipal Bond 50% Investment Grade/50% High Yield Index(reflects no deduction for fees, expenses or taxes)        
Average Annual Return, Percent   3.66% 1.55%   3.29%
S&P Intermediate Term National AMT-Free Municipal Bond Index (reflects no deduction for fees, expenses or taxes)          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] S&P Intermediate Term National AMT-Free Municipal Bond Index(reflects no deduction for fees, expenses or taxes)        
Average Annual Return, Percent   4.65% 1.22%   2.25%
HIGH-YIELD MUNICIPAL FUND, INVESTOR CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] Investor Class Return Before Taxes        
Average Annual Return, Percent   2.95% 1.11%   3.02%
Performance Inception Date Mar. 31, 1998        
HIGH-YIELD MUNICIPAL FUND, INVESTOR CLASS | After Taxes on Distributions          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions        
Average Annual Return, Percent   2.95% 1.07%   2.99%
Performance Inception Date Mar. 31, 1998        
HIGH-YIELD MUNICIPAL FUND, INVESTOR CLASS | After Taxes on Distributions and Sales          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions and Sale of Fund Shares        
Average Annual Return, Percent   3.53% 1.71%   3.19%
Performance Inception Date Mar. 31, 1998        
HIGH-YIELD MUNICIPAL FUND, I CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] I Class Return Before Taxes        
Average Annual Return, Percent   3.15% 1.31%   3.23%
Performance Inception Date Mar. 01, 2010        
HIGH-YIELD MUNICIPAL FUND, Y CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] Y Class Return Before Taxes        
Average Annual Return, Percent   3.07% 1.34% 3.29%  
Performance Inception Date Apr. 10, 2017        
HIGH-YIELD MUNICIPAL FUND, A CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] A Class Return Before Taxes        
Average Annual Return, Percent   (2.04%) (0.07%)   2.28%
Performance Inception Date Jan. 31, 2003        
HIGH-YIELD MUNICIPAL FUND, C CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] [1] C Class1 Return Before Taxes        
Average Annual Return, Percent [1]   1.81% 0.10%   2.15%
Performance Inception Date [1] Jul. 24, 2002        
INTERMEDIATE-TERM TAX-FREE BOND FUND, INVESTOR CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] Investor Class Return Before Taxes        
Average Annual Return, Percent   4.52% 1.03%   2.17%
Performance Inception Date Mar. 02, 1987        
INTERMEDIATE-TERM TAX-FREE BOND FUND, INVESTOR CLASS | After Taxes on Distributions          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions        
Average Annual Return, Percent   4.52% 1.03%   2.17%
Performance Inception Date Mar. 02, 1987        
INTERMEDIATE-TERM TAX-FREE BOND FUND, INVESTOR CLASS | After Taxes on Distributions and Sales          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] Return After Taxes on Distributions and Sale of Fund Shares        
Average Annual Return, Percent   4.08% 1.43%   2.29%
Performance Inception Date Mar. 02, 1987        
INTERMEDIATE-TERM TAX-FREE BOND FUND, I CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] I Class Return Before Taxes        
Average Annual Return, Percent   4.73% 1.23%   2.37%
Performance Inception Date Apr. 15, 2003        
INTERMEDIATE-TERM TAX-FREE BOND FUND, Y CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] Y Class Return Before Taxes        
Average Annual Return, Percent   4.86% 1.28% 2.56%  
Performance Inception Date Apr. 10, 2017        
INTERMEDIATE-TERM TAX-FREE BOND FUND, A CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] A Class Return Before Taxes        
Average Annual Return, Percent   (0.43%) (0.15%)   1.45%
Performance Inception Date Mar. 01, 2010        
INTERMEDIATE-TERM TAX-FREE BOND FUND, C CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] [2] C Class1 Return Before Taxes        
Average Annual Return, Percent [2]   3.58% 0.04%   1.30%
Performance Inception Date [2] Mar. 01, 2010        
TAX-FREE MONEY MARKET FUND, INVESTOR CLASS          
Prospectus [Line Items]          
Average Annual Return, Label [Optional Text] Investor Class        
Average Annual Return, Percent   2.36% 1.85%   1.22%
[1] C Class shares automatically convert to A Class shares after approximately eight years. All returns for periods greater than eight years reflect this conversion.
[2] C Class shares automatically convert to A Class shares after approximately eight years. All returns for periods greater than eight years reflect this conversion.