v3.25.2
Shareholder Report, Line Graph (Details) - USD ($)
 12 Months Ended 24 Months Ended 36 Months Ended 48 Months Ended 60 Months Ended 72 Months Ended 84 Months Ended 96 Months Ended 108 Months Ended 120 Months Ended
Jun. 30, 2015
Jun. 30, 2025
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2025
C000032726 [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   Alabama Tax‑Free Income Series                    
Accumulated Value $ 10,000   $ 10,661 $ 10,573 $ 10,705 $ 11,326 $ 11,948 $ 12,298 $ 11,311 $ 11,432 $ 11,795 $ 12,590
C000032727 [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   Kentucky Tax‑Free Income Series                    
Accumulated Value 10,000   10,625 10,551 10,649 11,312 11,660 12,200 11,210 11,473 11,821 12,636
C000032729 [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   Mississippi Tax‑Free Income Series                    
Accumulated Value 10,000   10,749 10,697 10,803 11,375 11,831 12,148 11,242 11,373 11,668 12,341
C000032730 [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   North Carolina Tax‑Free Income Series                    
Accumulated Value 10,000   10,728 10,548 10,682 11,289 11,796 12,164 11,067 11,288 11,614 12,282
C000032732 [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   Tennessee Tax‑Free Income Series                    
Accumulated Value 10,000   10,640 10,530 10,629 11,173 11,595 11,924 11,099 11,297 11,615 12,253
C000032734 [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   Intermediate Government Bond Series                    
Accumulated Value 10,000   10,657 10,446 10,336 10,865 11,395 11,340 10,971 11,014 11,519 12,079
C000093320 [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   Taxable Municipal Bond Series                    
Accumulated Value 10,000   10,929 11,014 11,307 12,057 12,684 13,029 10,572 10,320 10,552 10,974
Bloomberg Municipal Bond Index [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   Bloomberg Municipal Bond Index                    
Accumulated Value 10,000   10,765 10,713 10,880 11,610 12,126 12,640 11,557 11,925 12,308 12,445
Bloomberg Intermediate U S Government Bond Index [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   Bloomberg Intermediate U.S. Government Bond Index                    
Bloomberg Taxable Municipal Bond Index [Member]                        
Account Value [Line Items]                        
Line Graph and Table Measure Name   Bloomberg Taxable Municipal Bond Index                    
Bloomberg US Government Intermediate Index [Member]                        
Account Value [Line Items]                        
Accumulated Value 10,000   10,393 10,263 10,189 10,816 11,574 11,443 10,719 10,613 10,974 11,661
Bloomberg Municipal Index Taxable Bonds [Member]                        
Account Value [Line Items]                        
Accumulated Value $ 10,000   $ 11,271 $ 11,204 $ 11,501 $ 12,681 $ 13,913 $ 14,395 $ 12,463 $ 12,426 $ 12,900 $ 13,615