Risk/Return Detail Data - FidelityAdvisorMidCapIIFund-AMCIZPRO - USD ($)			12 Months Ended											60 Months Ended	120 Months Ended
		Feb. 28, 2026	Feb. 28, 2026	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 28, 2026	Feb. 28, 2026
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund | Fidelity Advisor Mid Cap II Fund - Class A
Prospectus Line Items
Maximum Cumulative Sales Charge (as a percentage of Offering Price)		5.75%
Maximum Deferred Sales Charge (as a percentage)	[1]	0.00%
Management Fees (as a percentage of Assets)		0.69%
Distribution and Service (12b-1) Fees		0.25%
Other Expenses (as a percentage of Assets):		0.02%
Expenses (as a percentage of Assets)		0.96%
Expense Example, with Redemption, 1 Year		$ 667
Expense Example, with Redemption, 3 Years		863
Expense Example, with Redemption, 5 Years		1,075
Expense Example, with Redemption, 10 Years		1,685
Expense Example, No Redemption, 1 Year		667
Expense Example, No Redemption, 3 Years		863
Expense Example, No Redemption, 5 Years		1,075
Expense Example, No Redemption, 10 Years		$ 1,685
Average Annual Return, Label [Optional Text]			Return Before Taxes
Average Annual Return, Percent			5.10%											8.39%	9.54%
Annual Return [Percent]				11.51%	16.92%	14.70%	(15.02%)	24.92%	18.34%	23.31%	(15.10%)	20.25%	11.59%
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund | Fidelity Advisor Mid Cap II Fund - Class M
Prospectus Line Items
Maximum Cumulative Sales Charge (as a percentage of Offering Price)		3.50%
Maximum Deferred Sales Charge (as a percentage)	[1]	0.00%
Management Fees (as a percentage of Assets)		0.69%
Distribution and Service (12b-1) Fees		0.50%
Other Expenses (as a percentage of Assets):		0.02%
Expenses (as a percentage of Assets)		1.21%
Expense Example, with Redemption, 1 Year		$ 469
Expense Example, with Redemption, 3 Years		721
Expense Example, with Redemption, 5 Years		992
Expense Example, with Redemption, 10 Years		1,765
Expense Example, No Redemption, 1 Year		469
Expense Example, No Redemption, 3 Years		721
Expense Example, No Redemption, 5 Years		992
Expense Example, No Redemption, 10 Years		$ 1,765
Average Annual Return, Label [Optional Text]			Return Before Taxes
Average Annual Return, Percent			7.36%											8.64%	9.53%
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund | Fidelity Advisor Mid Cap II Fund - Class C
Prospectus Line Items
Maximum Cumulative Sales Charge (as a percentage of Offering Price)		0.00%
Maximum Deferred Sales Charge (as a percentage)	[2]	1.00%
Management Fees (as a percentage of Assets)		0.69%
Distribution and Service (12b-1) Fees		1.00%
Other Expenses (as a percentage of Assets):		0.02%
Expenses (as a percentage of Assets)		1.71%
Expenses Deferred Charges [Text Block]		On Class C shares redeemed less than one year after purchase.
Expense Example, with Redemption, 1 Year		$ 274
Expense Example, with Redemption, 3 Years		539
Expense Example, with Redemption, 5 Years		928
Expense Example, with Redemption, 10 Years		1,821
Expense Example, No Redemption, 1 Year		174
Expense Example, No Redemption, 3 Years		539
Expense Example, No Redemption, 5 Years		928
Expense Example, No Redemption, 10 Years		$ 1,821
Average Annual Return, Label [Optional Text]			Return Before Taxes
Average Annual Return, Percent			9.68%											8.82%	9.49%
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund | Fidelity Advisor Mid Cap II Fund - Class I
Prospectus Line Items
Maximum Cumulative Sales Charge (as a percentage of Offering Price)		0.00%
Maximum Deferred Sales Charge (as a percentage)		0.00%
Management Fees (as a percentage of Assets)		0.69%
Distribution and Service (12b-1) Fees		0.00%
Other Expenses (as a percentage of Assets):		0.01%
Expenses (as a percentage of Assets)		0.70%
Expense Example, with Redemption, 1 Year		$ 72
Expense Example, with Redemption, 3 Years		224
Expense Example, with Redemption, 5 Years		390
Expense Example, with Redemption, 10 Years		871
Expense Example, No Redemption, 1 Year		72
Expense Example, No Redemption, 3 Years		224
Expense Example, No Redemption, 5 Years		390
Expense Example, No Redemption, 10 Years		$ 871
Average Annual Return, Label [Optional Text]			Return Before Taxes
Average Annual Return, Percent			11.81%											9.97%	10.49%
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund | Fidelity Advisor Mid Cap II Fund - Class Z
Prospectus Line Items
Maximum Cumulative Sales Charge (as a percentage of Offering Price)		0.00%
Maximum Deferred Sales Charge (as a percentage)		0.00%
Management Fees (as a percentage of Assets)		0.56%
Distribution and Service (12b-1) Fees		0.00%
Other Expenses (as a percentage of Assets):		0.02%
Expenses (as a percentage of Assets)		0.58%
Expense Example, with Redemption, 1 Year		$ 59
Expense Example, with Redemption, 3 Years		186
Expense Example, with Redemption, 5 Years		324
Expense Example, with Redemption, 10 Years		726
Expense Example, No Redemption, 1 Year		59
Expense Example, No Redemption, 3 Years		186
Expense Example, No Redemption, 5 Years		324
Expense Example, No Redemption, 10 Years		$ 726
Average Annual Return, Label [Optional Text]			Return Before Taxes
Average Annual Return, Percent			11.96%											10.11%	10.64%
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund
Prospectus Line Items
Risk/Return [Heading]		Fund Summary Fund /Class: Fidelity Advisor® Mid Cap II Fund /A, M, C, I, Z
Objective [Heading]		Investment Objective
Objective, Primary [Text Block]		Fidelity Advisor® Mid Cap II Fund seeks long-term growth of capital.
Expense Heading [Optional Text]		Fee Table
Expense Narrative [Text Block]		The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $ 50,000 in the fund or certain other Fidelity ® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 31 of the prospectus and in the sales charge waiver appendix to the prospectus.
Shareholder Fees Caption [Optional Text]		Shareholder fees
Operating Expenses Caption [Optional Text]		Annual Operating Expenses
Portfolio Turnover [Heading]		Portfolio Turnover
Portfolio Turnover [Text Block]		The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 62 % of the average value of its portfolio.
Portfolio Turnover, Rate		62.00%
Expenses Deferred Charges [Text Block]		Class A and Class M purchases of $1 million or more will not be subject to a front-end sales charge. Such Class A and Class M purchases may be subject, upon redemption, to a contingent deferred sales charge (CDSC) of 1.00% and 0.25%, respectively.
Expense Breakpoint Discounts [Text]		You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $ 50,000 in the fund or certain other Fidelity ® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 31 of the prospectus
Expense Breakpoint, Minimum Investment Required [Amount]		$ 50,000
Expense Example Narrative [Text Block]		This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated and if you hold your shares:
Strategy [Heading]		Principal Investment Strategies
Strategy Narrative [Text Block]		Normally investing at least 80% of assets in securities of companies with medium market capitalizations. The Adviser generally defines medium market capitalization companies as those whose market capitalization is similar to the market capitalization of companies in the Russell Midcap ® Index or the S&P MidCap 400 ® Index. Derivative instruments that provide investment exposure to the investments above or exposure to one or more market risk factors associated with such investments are included in the fund's 80% policy, consistent with the fund's investment policies and limitations with respect to investments in derivatives. Normally investing primarily in common stocks. Potentially investing in companies with smaller or larger market capitalizations. Investing in domestic and foreign issuers. Investing in either "growth" stocks or "value" stocks or both. Using fundamental analysis of factors such as each issuer's financial condition and industry position, as well as market and economic conditions, to select investments.
Bar Chart and Performance Table [Heading]		Performance
Performance Narrative [Text Block]		The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time. The additional index has characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.
Performance Information Illustrates Variability of Returns [Text]		The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time.
Performance Availability Website Address [Text]		institutional.fidelity.com
Performance Past Does Not Indicate Future [Text]		Past performance (before and after taxes) is not an indication of future performance.
Bar Chart [Heading]		Year-by-Year Returns
Bar Chart Narrative [Text Block]		The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.
Bar Chart Does Not Reflect Sales Loads [Text]		The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.
Highest Quarterly Return, Label [Optional Text]		Highest Quarter Return
Highest Quarterly Return, Date		Jun. 30, 2020
Highest Quarterly Return		25.11%
Lowest Quarterly Return, Label [Optional Text]		Lowest Quarter Return
Lowest Quarterly Return, Date		Mar. 31, 2020
Lowest Quarterly Return		(29.88%)
Performance Table Heading		Average Annual Returns
Performance Table Does Reflect Sales Loads		Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges.
Performance Table Uses Highest Federal Rate		After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes.
Performance Table Not Relevant to Tax Deferred		The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan).
Performance Table One Class of after Tax Shown [Text]		After-tax returns for Class A are shown in the table below and after-tax returns for other classes will vary.
Performance Table Explanation after Tax Higher		Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.
Performance Table Narrative		Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges. After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. After-tax returns for Class A are shown in the table below and after-tax returns for other classes will vary. Actual after-tax returns may differ depending on your individual circumstances. The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan). Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund | After Taxes on Distributions | Fidelity Advisor Mid Cap II Fund - Class A
Prospectus Line Items
Average Annual Return, Label [Optional Text]			Return After Taxes on Distributions
Average Annual Return, Percent			2.69%											6.27%	7.80%
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund | After Taxes on Distributions and Sales | Fidelity Advisor Mid Cap II Fund - Class A
Prospectus Line Items
Average Annual Return, Label [Optional Text]			Return After Taxes on Distributions and Sale of Fund Shares
Average Annual Return, Percent			4.69%											6.32%	7.41%
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund | SP004
Prospectus Line Items
Average Annual Return, Label [Optional Text]			S&P MidCap 400® Index
Average Annual Return, Percent			7.50%											9.12%	10.72%
FidelityAdvisorMidCapIIFund-AMCIZPRO | Fidelity Advisor Mid Cap II Fund | RS003
Prospectus Line Items
Average Annual Return, Label [Optional Text]			Russell 3000® Index
Average Annual Return, Percent			17.15%											13.15%	14.29%
Document Type		485BPOS
Registrant Name		Fidelity Advisor Series I

[1] A Class A and Class M purchases of $1 million or more will not be subject to a front-end sales charge. Such Class A and Class M purchases may be subject, upon redemption, to a contingent deferred sales charge (CDSC) of 1.00% and 0.25%, respectively.

[2] B On Class C shares redeemed less than one year after purchase.