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Shareholder Report
 6 Months Ended
May 31, 2026
USD ($)
shares
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Fidelity Advisor Series IV
Entity Central Index Key 0000719451
Entity Investment Company Type N-1A
Document Period End Date May 31, 2026
Fidelity Limited Term Government Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Limited Term Government Fund
Class Name Fidelity® Limited Term Government Fund
Trading Symbol FFXSX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Limited Term Government Fund for the period December 1, 2025 to May 31, 2026.
Shareholder Report Annual or Semi-Annual semi-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 six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Limited Term Government Fund 
$ 15 
0.30%
  
Expenses Paid, Amount $ 15
Expense Ratio, Percent 0.30%
Net Assets $ 159,676,648
Holdings Count | shares 226
Investment Company Portfolio Turnover 113.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of May 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$159,676,648
  
Number of Holdings
226
  
Portfolio Turnover
113%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 1 - 1.99% 0.1   2 - 2.99% 4.6   3 - 3.99% 67.8   4 - 4.99% 24.9   5 - 5.99% 0.6   6 - 6.99% 2.1         U.S. Treasury Obligations 78.5 CMOs and Other Mortgage Related Securities 15.3 U.S. Government Agency Obligations 3.2 U.S. Government Agency - Mortgage Securities 3.1 Options 0.4 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 78.5           CMOs and Other Mortgage Related Securities - 15.3   U.S. Government Agency Obligations - 3.2   U.S. Government Agency - Mortgage Securities - 3.1   Options - 0.4                            Short-Term Investments and Net Other Assets (Liabilities) - (0.5)%  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Notes 77.5   Freddie Mac Multifamily Structured pass-thru certificates 11.2   Federal Farm Credit Banks Funding Corp 3.2   Freddie Mac Gold Pool 2.5   Fannie Mae Mortgage pass-thru certificates 1.7   Ginnie Mae II Pool 1.1   Fannie Mae Guaranteed REMIC 1.1   US Treasury Bonds 1.0   Ginnie Mae Mortgage pass-thru certificates 0.5   Ginnie Mae REMIC pass-thru certificates 0.1     99.9