| Form N-CSR Cover | 12 Months Ended |
| Nov. 30, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | Fidelity Advisor Series IV |
| Entity Central Index Key | 0000719451 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Nov. 30, 2024 |
| Shareholder Report | 12 Months Ended |
| Nov. 30, 2024 USD ($) shares | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | Fidelity Advisor Series IV |
| Entity Central Index Key | 0000719451 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Nov. 30, 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 annual shareholder report contains information about Fidelity® Limited Term Government Fund for the period December 1, 2023 to November 30, 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® Limited Term Government Fund $ 0.30 % |
| Expenses Paid, Amount | $ 31 |
| Expense Ratio, Percent | 0.30% |
| Factors Affecting Performance [Text Block] | What affected the Fund's performance this period? • • • • • |
| Performance Past Does Not Indicate Future [Text] | The Fund's past performance is not a good predictor of the Fund's future performance.</span>" id="sjs-B26"><span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">The Fund's past performance is not a good predictor of the Fund's future performance.</span> |
| Line Graph [Table Text Block] | How did the Fund perform over the past 10 years? CUMULATIVE PERFORMANCE November 30, 2014 through November 30, 2024. Initial investment of $10,000. Fidelity® Limited Term Government Fund $10,000 $10,053 $10,090 $10,138 $10,158 $10,643 $11,057 $10,920 $10,225 $10,474 $10,982 Bloomberg U.S. 1-5 Year Government Bond Index $10,000 $10,076 $10,163 $10,235 $10,275 $10,815 $11,292 $11,193 $10,554 $10,852 $11,396 Bloomberg U.S. Aggregate Bond Index $10,000 $10,097 $10,316 $10,648 $10,505 $11,639 $12,486 $12,342 $10,758 $10,885 $11,633 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 |
| Average Annual Return [Table Text Block] | AVERAGE ANNUAL TOTAL RETURNS: 1 Year 5 Year 10 Year Fidelity® Limited Term Government Fund 4.85% 0.63% 0.94% Bloomberg U.S. 1-5 Year Government Bond Index 5.01% 1.05% 1.32% Bloomberg U.S. Aggregate Bond Index 6.88% -0.01% 1.52% Visit www.fidelity.com for more recent performance information. The Fund's past performance is not a good predictor of the Fund's future performance. The graph and table do not reflect the deduction of taxes that a shareholder would pay on fund distributions or redemption of fund shares. |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on fund distributions or redemption of fund shares. |
| Net Assets | $ 196,928,167 |
| Holdings Count | shares | 239 |
| Advisory Fees Paid, Amount | $ 469,234 |
| Investment Company Portfolio Turnover | 75% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics (as of November 30, 2024) KEY FACTS Fund Size $ Number of Holdings 239 Total Advisory Fee $ Portfolio Turnover 75 % |
| Holdings [Text Block] | COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 4.5 1 - 1.99% 6.9 2 - 2.99% 18.0 3 - 3.99% 26.5 4 - 4.99% 35.5 5 - 5.99% 2.4 6 - 6.99% 0.7 7 - 7.99% 0.0 8 - 8.99% 0.0 U.S. Treasury Obligations 71.8 CMOs and Other Mortgage Related Securities 18.4 U.S. Government Agency - Mortgage Securities 4.3 Purchased Options 0.2 Short-Term Investments and Net Other Assets (Liabilities) 5.3 ASSET ALLOCATION (% of Fund's net assets) |
| Largest Holdings [Text Block] | TOP HOLDINGS (% of Fund's net assets) US Treasury Notes 71.8 Freddie Mac Multifamily Structured pass-thru certificates 14.2 Fannie Mae Guaranteed REMICS 1.7 Fannie Mae Mortgage pass-thru certificates 1.7 Uniform Mortgage Backed Securities 1.6 FREMF Mortgage Trust 1.2 Freddie Mac Gold Pool 0.9 Ginnie Mae Mortgage pass-thru certificates 0.8 Freddie Mac Multiclass Mortgage participation certificates 0.3 Ginnie Mae REMIC pass-thru certificates 0.1 94.3 |
| Material Fund Change [Text Block] | How has the Fund changed? This is a summary of certain changes to the Fund since December 1, 2023. For more complete information, you may review the Fund's next prospectus, which we expect to be available by January 29, 2025 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 fees associated with this class changed during the reporting year. The variations in class fees are primarily the result of the following changes: Management fee Operating expenses |
| Material Fund Change Expenses [Text Block] | The fees associated with this class changed during the reporting year. The variations in class fees are primarily the result of the following changes: Management fee Operating expenses |
| Summary of Change Legend [Text Block] | This is a summary of certain changes to the Fund since December 1, 2023. For more complete information, you may review the Fund's next prospectus, which we expect to be available by January 29, 2025 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-B40"><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-B41"><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-B42"><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, Line Graph (Details) - USD ($) | 12 Months Ended | 60 Months Ended | 120 Months Ended | 132 Months Ended | ||||||||||
| Nov. 30, 2024 | Nov. 30, 2024 | Nov. 30, 2024 | Nov. 30, 2024 | Nov. 30, 2023 | Nov. 30, 2022 | Nov. 30, 2021 | Nov. 30, 2020 | Nov. 30, 2019 | Nov. 30, 2018 | Nov. 30, 2017 | Nov. 30, 2016 | Nov. 30, 2015 | Nov. 30, 2014 | |
| C000014122_1_BBIndexMember_1YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Average Annual Return, Percent | 6.88% | |||||||||||||
| C000014122_1_BBIndexMember_5YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Average Annual Return, Percent | (0.01%) | |||||||||||||
| C000014122_1_BBIndexMember_10YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Average Annual Return, Percent | 1.52% | |||||||||||||
| C000014122_LG_ClassMember_11YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Line Graph and Table Measure Name | Fidelity® Limited Term Government Fund</span>" id="sjs-E14"><span style="font-family:'Fidelity Sans';font-size:9px;color:#000000;font-style:normal;font-weight:normal;">Fidelity® Limited Term Government Fund</span> | |||||||||||||
| C000014122_ClassMember_AsOf_2014Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,000 | |||||||||||||
| C000014122_ClassMember_AsOf_2015Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,053 | |||||||||||||
| C000014122_ClassMember_AsOf_2016Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,090 | |||||||||||||
| C000014122_ClassMember_AsOf_2017Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,138 | |||||||||||||
| C000014122_ClassMember_AsOf_2018Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,158 | |||||||||||||
| C000014122_ClassMember_AsOf_2019Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,643 | |||||||||||||
| C000014122_ClassMember_AsOf_2020Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 11,057 | |||||||||||||
| C000014122_ClassMember_AsOf_2021Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,920 | |||||||||||||
| C000014122_ClassMember_AsOf_2022Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,225 | |||||||||||||
| C000014122_ClassMember_AsOf_2023Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,474 | |||||||||||||
| C000014122_ClassMember_AsOf_2024Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,982 | $ 10,982 | $ 10,982 | $ 10,982 | ||||||||||
| C000014122_LG_1_AdditionalIndexMember_11YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. 1-5 Year Government Bond Index</span>" id="sjs-E50"><span style="font-family:'Fidelity Sans';font-size:9px;color:#000000;font-style:normal;font-weight:normal;">Bloomberg U.S. 1-5 Year Government Bond Index</span> | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2014Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 10,000 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2015Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 10,076 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2016Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 10,163 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2017Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 10,235 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2018Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 10,275 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2019Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 10,815 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2020Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 11,292 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2021Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 11,193 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2022Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 10,554 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2023Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 10,852 | |||||||||||||
| C000014122_1_AdditionalIndexMember_AsOf_2024Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | 11,396 | 11,396 | 11,396 | $ 11,396 | ||||||||||
| C000014122_LG_2_AdditionalIndexMember_11YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Line Graph and Table Measure Name | Bloomberg U.S. Aggregate Bond Index</span>" id="sjs-E86"><span style="font-family:'Fidelity Sans';font-size:9px;color:#000000;font-style:normal;font-weight:normal;">Bloomberg U.S. Aggregate Bond Index</span> | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2014Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,000 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2015Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,097 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2016Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,316 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2017Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,648 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2018Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,505 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2019Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 11,639 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2020Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 12,486 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2021Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 12,342 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2022Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,758 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2023Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 10,885 | |||||||||||||
| C000014122_2_AdditionalIndexMember_AsOf_2024Member | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Account Value | $ 11,633 | $ 11,633 | $ 11,633 | $ 11,633 | ||||||||||
| C000014122_NoLoadClassMember_1YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Average Annual Return, Percent | 4.85% | |||||||||||||
| C000014122_NoLoadClassMember_5YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Average Annual Return, Percent | 0.63% | |||||||||||||
| C000014122_NoLoadClassMember_10YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Average Annual Return, Percent | 0.94% | |||||||||||||
| C000014122_0_AdditionalIndexMember_1YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Average Annual Return, Percent | 5.01% | |||||||||||||
| C000014122_0_AdditionalIndexMember_5YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Average Annual Return, Percent | 1.05% | |||||||||||||
| C000014122_0_AdditionalIndexMember_10YRMember | ||||||||||||||
| Account Value [Line Items] | ||||||||||||||
| Average Annual Return, Percent | 1.32% |
| Shareholder Report, Holdings (Details) - Fidelity Limited Term Government Fund | Nov. 30, 2024 |
| TH0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 71.80% |
| TH1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 14.20% |
| TH2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| TH3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.70% |
| TH4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| TH5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| TH6Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.90% |
| TH7Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| TH8Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.30% |
| TH9Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.10% |
| CD0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.50% |
| CD1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.90% |
| CD2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18% |
| CD3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 26.50% |
| CD4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 35.50% |
| CD5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| CD6Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.70% |
| 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 | 71.80% |
| AACMOsandOtherMortgageRelatedSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.40% |
| AAUSGovernmentAgencyMortgageSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.30% |
| AAPurchasedOptionsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.30% |
