Risk/Return Detail Data - FidelityMunicipalMoneyMarketFund-PRO - USD ($) |
12 Months Ended | 60 Months Ended | 120 Months Ended | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Oct. 30, 2026 |
Oct. 30, 2026 |
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 |
Oct. 30, 2026 |
Oct. 30, 2026 |
|
| FidelityMunicipalMoneyMarketFund-PRO | Fidelity Municipal Money Market Fund | Fidelity Municipal Money Market Fund | ||||||||||||||
| Prospectus Line Items | ||||||||||||||
| Management Fees (as a percentage of Assets) | 0.38% | |||||||||||||
| Distribution and Service (12b-1) Fees | 0.00% | |||||||||||||
| Other Expenses (as a percentage of Assets): | 0.01% | |||||||||||||
| Expenses (as a percentage of Assets) | 0.39% | |||||||||||||
| Expense Example, with Redemption, 1 Year | $ 40 | |||||||||||||
| Expense Example, with Redemption, 3 Years | 125 | |||||||||||||
| Expense Example, with Redemption, 5 Years | 219 | |||||||||||||
| Expense Example, with Redemption, 10 Years | $ 493 | |||||||||||||
| Average Annual Return, Label [Optional Text] | Fidelity® Municipal Money Market Fund | |||||||||||||
| Average Annual Return, Percent | 2.49% | 1.93% | 1.30% | |||||||||||
| Annual Return [Percent] | 2.49% | 3.14% | 3.13% | 0.92% | 0.02% | 0.38% | 1.24% | 1.12% | 0.52% | 0.15% | ||||
| FidelityMunicipalMoneyMarketFund-PRO | Fidelity Municipal Money Market Fund | ||||||||||||||
| Prospectus Line Items | ||||||||||||||
| Risk/Return [Heading] | Fund Summary Fund: Fidelity® Municipal Money Market Fund | |||||||||||||
| Objective [Heading] | Investment Objective | |||||||||||||
| Objective, Primary [Text Block] | Fidelity® Municipal Money Market Fund seeks as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal. | |||||||||||||
| Expense Heading [Optional Text] | Fee Table | |||||||||||||
| Expense Narrative [Text Block] | The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions, which are not reflected in the tables and examples below. | |||||||||||||
| Shareholder Fees Caption [Optional Text] | Shareholder fees | |||||||||||||
| Shareholder Fee, Other | $ 0 | |||||||||||||
| Operating Expenses Caption [Optional Text] | Annual Operating Expenses | |||||||||||||
| Expense Example Narrative [Text Block] | This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated: | |||||||||||||
| Strategy [Heading] | Principal Investment Strategies | |||||||||||||
| Strategy Narrative [Text Block] | Normally investing in municipal money market securities. Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax. Municipal money market securities are high-quality, short-term debt securities that pay a fixed, variable, or floating interest rate, and include variable rate demand notes, commercial paper, and municipal notes. Municipal securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Potentially investing up to 20% of assets in securities subject to state and/or federal income tax. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. | |||||||||||||
| Bar Chart and Performance Table [Heading] | Performance | |||||||||||||
| Performance Narrative [Text Block] | The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance. Visit www.fidelity.com for more recent performance information. | |||||||||||||
| Performance Information Illustrates Variability of Returns [Text] | The information illustrates the changes in the performance of the fund's shares from year to year. | |||||||||||||
| Performance Availability Website Address [Text] | www.fidelity.com | |||||||||||||
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future performance. | |||||||||||||
| Bar Chart [Heading] | Year-by-Year Returns | |||||||||||||
| Year to Date Return, Label [Optional Text] | Year-to-Date Return | |||||||||||||
| Bar Chart, Year to Date Return, Date | Sep. 30, 2026 | |||||||||||||
| Bar Chart, Year to Date Return | 1.58% | |||||||||||||
| Highest Quarterly Return, Label [Optional Text] | Highest Quarter Return | |||||||||||||
| Highest Quarterly Return, Date | Dec. 31, 2023 | |||||||||||||
| Highest Quarterly Return | 0.90% | |||||||||||||
| Lowest Quarterly Return, Label [Optional Text] | Lowest Quarter Return | |||||||||||||
| Lowest Quarterly Return, Date | Mar. 31, 2021 | |||||||||||||
| Lowest Quarterly Return | none | |||||||||||||
| Performance Table Heading | Average Annual Returns | |||||||||||||
| Document Type | 485BPOS | |||||||||||||
| Registrant Name | Fidelity Union Street Trust II | |||||||||||||