| Form N-CSR Cover | 6 Months Ended |
| May 31, 2024 | |
| 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 |
| 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 $ 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% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics (as of May 31, 2024) KEY FACTS Fund Size $ 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 |
| Shareholder Report, Holdings (Details) - Fidelity Limited Term Government Fund | May 31, 2024 |
| TH0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 73.40% |
| TH1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 9.10% |
| TH2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.80% |
| TH3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
| TH4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.50% |
| TH5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| TH6Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| TH7Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.70% |
| TH8Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| TH9Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| CD0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 22.10% |
| CD1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.70% |
| CD2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.10% |
| CD3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.10% |
| CD4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 30.80% |
| CD5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2% |
| CD6Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.60% |
| CD7Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0% |
| CD8Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0% |
| AAUSTreasuryObligationsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 74.60% |
| AACMOsandOtherMortgageRelatedSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.30% |
| AAUSGovernmentAgencyMortgageSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.10% |
| AACorporateBondsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.60% |
