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Shareholder Report
 6 Months Ended
May 31, 2024
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, 2024
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, 2023 to May 31, 2024.
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 $ 233,596,758
Holdings Count | shares 300
Investment Company Portfolio Turnover 84.00%
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of May 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$233,596,758
 
  
Number of Holdings
300
 
  
Portfolio Turnover
84%
 
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 22.1     1 - 1.99% 7.7     2 - 2.99% 16.1     3 - 3.99% 17.1     4 - 4.99% 30.8     5 - 5.99% 2.0     6 - 6.99% 0.6     7 - 7.99% 0.0     8 - 8.99% 0.0       U.S. Treasury Obligations 74.6 CMOs and Other Mortgage Related Securities 15.3 U.S. Government Agency - Mortgage Securities 6.1 Corporate Bonds 0.4 Short-Term Investments and Net Other Assets (Liabilities) 3.6 ASSET ALLOCATION (% of Fund's net assets)
Largest Holdings [Text Block] TOP HOLDINGS (% of Fund's net assets)     US Treasury Notes 73.4     Freddie Mac Multifamily Structured Pass Through Certificates 9.1     Fannie Mae Mortgage pass-thru certificates 3.8     Fannie Mae Guaranteed REMICS 3.6     Freddie Mac Gold Pool 2.5     US Treasury Bonds 1.2     Fremf 2015-Kplb Mortgage Trust 1.0     Ginnie Mae Mortgage pass-thru certificates 0.7     Private Export Funding Corp 0.4     Freddie Mac Multiclass Mortgage participation certificates 0.4       96.1