| Form N-CSR Cover | 12 Months Ended |
| Aug. 31, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | Fidelity Union Street Trust II |
| Entity Central Index Key | 0000880797 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Aug. 31, 2024 |
| Shareholder Report | 12 Months Ended |
| Aug. 31, 2024 USD ($) shares | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | Fidelity Union Street Trust II |
| Entity Central Index Key | 0000880797 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Aug. 31, 2024 |
| Fidelity Municipal Money Market Fund | |
| Shareholder Report [Line Items] | |
| Fund Name | Fidelity® Municipal Money Market Fund |
| Class Name | Fidelity® Municipal Money Market Fund |
| Trading Symbol | FTEXX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains information about Fidelity® Municipal Money Market Fund for the period September 1, 2023 to August 31, 2024. |
| Shareholder Report Annual or Semi-Annual | annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com. |
| Material Fund Change Notice [Text Block] | This report describes changes to the Fund that occurred during the reporting period. |
| Additional Information Phone Number | 1-800-544-8544 |
| Additional Information Email | fidfunddocuments@fidelity.com |
| Additional Information Website | fundresearch.fidelity.com/prospectus/sec |
| Expenses [Text Block] | What were your Fund costs for the last year? (based on hypothetical $10,000 investment) FUND COST (PREVIOUS YEAR) Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment Fidelity® Municipal Money Market Fund $ 0.40 % |
| Expenses Paid, Amount | $ 41 |
| Expense Ratio, Percent | 0.40% |
| Net Assets | $ 3,801,153,223 |
| Holdings Count | shares | 486 |
| Advisory Fees Paid, Amount | $ 12,052,106 |
| Additional Fund Statistics [Text Block] | Key Fund Statistics (as of August 31, 2024) KEY FACTS Fund Size $ Number of Holdings 486 Total Advisory Fee $ |
| Holdings [Text Block] | EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets) Days 1-7 78.3 8-30 5.1 31-60 3.7 61-90 1.5 91-180 0.5 >180 10.3 Variable Rate Demand Notes (VRDNs) 42.1 Tender Option Bond 32.0 Municipal Securities 15.7 Commercial Paper 5.3 Net Other Assets (Liabilities) 4.9 ASSET ALLOCATION (% of Fund's net assets) |
| Material Fund Change [Text Block] | How has the Fund changed? This is a summary of certain changes to the Fund since September 1, 2023. For more complete information, you may review the Fund's next prospectus, which we expect to be available by October 30, 2024 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-800-544-8544 or by sending an e-mail to fidfunddocuments fidelity.com . The fund's transfer agent and pricing & bookkeeping fees were changed to a fixed rate effective December 1, 2023, through February 29, 2024, in anticipation of the transition to a new management fee structure. Effective March 1, 2024, the fund's management contract was amended to incorporate administrative services previously covered under separate services agreements (transfer agent and pricing & bookkeeping). The amended contract incorporates a management fee rate that may vary by class. The Adviser or an affiliate pays certain expenses of managing and operating the fund out of each class's management fee. The fund's principal investment risks were updated in connection with the implementation of recent rule amendments related to money market funds adopted by the SEC. |
| Material Fund Change Expenses [Text Block] | The fund's transfer agent and pricing & bookkeeping fees were changed to a fixed rate effective December 1, 2023, through February 29, 2024, in anticipation of the transition to a new management fee structure. Effective March 1, 2024, the fund's management contract was amended to incorporate administrative services previously covered under separate services agreements (transfer agent and pricing & bookkeeping). The amended contract incorporates a management fee rate that may vary by class. The Adviser or an affiliate pays certain expenses of managing and operating the fund out of each class's management fee. |
| Material Fund Change Risks Change [Text Block] | The fund's principal investment risks were updated in connection with the implementation of recent rule amendments related to money market funds adopted by the SEC. |
| Summary of Change Legend [Text Block] | This is a summary of certain changes to the Fund since September 1, 2023. For more complete information, you may review the Fund's next prospectus, which we expect to be available by October 30, 2024 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-800-544-8544 or by sending an e-mail to fidfunddocuments fidelity.com . |
| Updated Prospectus Phone Number | 1-800-544-8544</span>" id="sjs-B34"><span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">1-800-544-8544</span> |
| Updated Prospectus Email Address | fidfunddocuments@fidelity.com</span>" id="sjs-B35"><span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">fidfunddocuments@fidelity.com</span> |
| Updated Prospectus Web Address | fundresearch.fidelity.com/prospectus/sec</span>" id="sjs-B36"><span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:bold;">fundresearch.fidelity.com/prospectus/sec</span> |
| Shareholder Report, Holdings (Details) - Fidelity Municipal Money Market Fund | Aug. 31, 2024 |
| EMD0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 78.30% |
| EMD1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.10% |
| EMD2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.70% |
| EMD3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| EMD4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.50% |
| EMD5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.30% |
| AAVariableRateDemandNotesVRDNsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 42.10% |
| AATenderOptionBondMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 32% |
| AAMunicipalSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.70% |
| AACommercialPaperMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.30% |
| AANetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.90% |
