In seeking to achieve its investment objective, the Fund will, under normal circumstances, invest at least 80% of its net assets in income-producing equity securities, including dividend-paying common and preferred stocks. Income-producing securities are those that NTI believes will provide dividend yield, based on the security’s indicative yield. The Fund seeks to provide a high level of current income relative to its benchmark index.

In determining capital appreciation potential, NTI uses a proprietary quantitative ranking system that is designed to provide exposure to equity securities that exhibit quality characteristics. Beginning with a broad universe of highly liquid equity securities, NTI applies a proprietary quality score, which ranks each security based on its issuer’s profitability, management efficiency, and cash generation, to screen out the lowest quality securities based on their proprietary ranking. NTI then selects securities from the remaining universe that it believes will achieve the appropriate quality and yield exposures. NTI also performs a risk management analysis in which NTI seeks to measure and manage risk exposures at the security, sector and portfolio levels through portfolio diversification. Final purchase decisions are made based on the desired level of diversification and dividend yield.

The Fund may use derivatives such as stock index futures contracts to equitize cash and enhance portfolio liquidity. Derivatives instruments that provide investment exposure to investments in the Fund’s 80% investment policy and derivatives instruments that provide investment exposure to one or more of the market risk factors associated with such securities may be counted towards the Fund’s 80% investment policy. NTI will normally sell a security that it believes is no longer attractive based upon the evaluation criteria described above.

S000001269 [Member] Investment Strategy - INCOME EQUITY FUND	Mar. 31, 2026
Prospectus [Line Items]
Strategy [Heading]	PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block]	In seeking to achieve its investment objective, the Fund will, under normal circumstances, invest at least 80% of its net assets in income-producing equity securities, including dividend-paying common and preferred stocks. Income-producing securities are those that NTI believes will provide dividend yield, based on the security’s indicative yield. The Fund seeks to provide a high level of current income relative to its benchmark index. In determining capital appreciation potential, NTI uses a proprietary quantitative ranking system that is designed to provide exposure to equity securities that exhibit quality characteristics. Beginning with a broad universe of highly liquid equity securities, NTI applies a proprietary quality score, which ranks each security based on its issuer’s profitability, management efficiency, and cash generation, to screen out the lowest quality securities based on their proprietary ranking. NTI then selects securities from the remaining universe that it believes will achieve the appropriate quality and yield exposures. NTI also performs a risk management analysis in which NTI seeks to measure and manage risk exposures at the security, sector and portfolio levels through portfolio diversification. Final purchase decisions are made based on the desired level of diversification and dividend yield. The Fund may use derivatives such as stock index futures contracts to equitize cash and enhance portfolio liquidity. Derivatives instruments that provide investment exposure to investments in the Fund’s 80% investment policy and derivatives instruments that provide investment exposure to one or more of the market risk factors associated with such securities may be counted towards the Fund’s 80% investment policy. NTI will normally sell a security that it believes is no longer attractive based upon the evaluation criteria described above. From time to time the Fund may have a focused investment (i.e., investment exposure comprising more than 15% of its total assets) in one or more particular sectors. As of March 31, 2026, the Fund had a focused investment in the information technology sector.