v3.26.3
Shareholder Report
 12 Months Ended
Aug. 31, 2026
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, 2026
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, 2025 to August 31, 2026.
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.
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 
$ 39 
0.39%
  
Expenses Paid, Amount $ 39
Expense Ratio, Percent 0.39%
Net Assets $ 3,851,130,879
Holdings Count | shares 607
Advisory Fees Paid, Amount $ 14,873,572
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2026)
 
KEY FACTS
 
  
  
Fund Size
$3,851,130,879
  
Number of Holdings
607
  
Total Advisory Fee
$14,873,572
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 20.5   8-30 57.1   31-60 7.3   61-90 3.2   91-180 2.5   >180 10.3             Variable Rate Demand Notes (VRDNs) 34.7 Tender Option Bond 30.7 Municipal Securities 15.3 Commercial Paper 7.8 Net Other Assets (Liabilities) 11.5 ASSET ALLOCATION (% of Fund's net assets)     Variable Rate Demand Notes (VRDNs) - 34.7   Tender Option Bond - 30.7                  Municipal Securities - 15.3                Commercial Paper - 7.8                     Net Other Assets (Liabilities) - 11.5