S000010032 [Member] Annual Total Returns |
12 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2025 |
Dec. 31, 2024 |
Dec. 31, 2023 |
Dec. 31, 2022 |
Dec. 31, 2021 |
Dec. 31, 2020 |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| The National Tax-Free Intermediate Bond Fund | Shares | ||||||||||
| Prospectus [Line Items] | ||||||||||
| Annual Return [Percent] | 5.09% | 0.87% | 4.81% | (7.13%) | 0.40% | 4.67% | 6.62% | 0.80% | 4.81% | 0.05% |