Net Assets Investment Company, Portfolio Turnover Alabama Tax-Free Income Series Bloomberg Municipal Bond Index [Member] Investment Type [Axis] State and Local Mortgage/Housing Revenue [Member] University Consolidated Education and Building Revenue [Member] Miscellaneous Public Improvement [Member] Municipal Utility Revenue [Member] Other Assets Less Liabilities [Member] Prerefunded [Member] Public Facilities Revenue [Member] Refunding [Member] School Improvement [Member] Aaa/AAA [Member] Aa/AA [Member] A [Member] Not Rated [Member] Kentucky Tax-Free Income Series Ad Valorem Property [Member] Airport Revenue [Member] Turnpikes/Toll Road/Highway Revenue [Member] Hospital And Healthcare Revenue [Member] Mississippi Tax-Free Income Series Lease Revenue [Member] North Carolina Tax-Free Income Series Tennessee Tax-Free Income Series Intermediate Government Bond Series Bloomberg US Government Intermediate Index [Member] Bloomberg Intermediate U.S. Government Bond Index [Member] Federal Farm Credit [Member] Federal Home Loan Bank [Member] Federal Home Loan Mortgage Corporation [Member] Federal National Mortgage Corporation [Member] Aa1 [Member] Taxable Municipal Bond Series Bloomberg Municipal Index Taxable Bonds [Member] Bloomberg Taxable Municipal Bond Index [Member] Marina/Port Authority Revenue [Member]