Shareholder Report, Average Annual Return (Details)	12 Months Ended	60 Months Ended	120 Months Ended
Shareholder Report, Average Annual Return (Details)	Jun. 30, 2025	Jun. 30, 2025	Jun. 30, 2025
C000032726 [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	Alabama Tax‑Free Income Series
Average Annual Return, Percent	(1.13%)	(0.50%)	1.52%
C000032727 [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	Kentucky Tax‑Free Income Series
Average Annual Return, Percent	(0.18%)	0.23%	1.64%
C000032729 [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	Mississippi Tax‑Free Income Series
Average Annual Return, Percent	0.08%	(0.27%)	1.54%
C000032730 [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	North Carolina Tax‑Free Income Series
Average Annual Return, Percent	0.05%	(0.31%)	1.49%
C000032732 [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	Tennessee Tax‑Free Income Series
Average Annual Return, Percent	(0.97%)	(0.17%)	1.38%
C000032734 [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	Intermediate Government Bond Series
Average Annual Return, Percent	4.86%	1.16%	1.88%
C000093320 [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	Taxable Municipal Bond Series
Average Annual Return, Percent	4.00%	(2.81%)	0.91%
Bloomberg Municipal Bond Index [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	Bloomberg Municipal Bond Index
Average Annual Return, Percent	1.11%	0.51%	2.20%
Bloomberg Intermediate U S Government Bond Index [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	Bloomberg Intermediate U.S. Government Bond Index
Average Annual Return, Percent	6.23%	0.15%	1.55%
Bloomberg Taxable Municipal Bond Index [Member]
Average Annual Return [Line Items]
Line Graph and Table Measure Name	Bloomberg Taxable Municipal Bond Index
Average Annual Return, Percent	5.51%	(0.43%)	3.13%