Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA FUNDS SERIES TRUST I
Prospectus Date	rr_ProspectusDate	Sep. 12, 2019
Overseas SMA Completion Portfolio
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	SUMMARY OF THE FUND
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Overseas SMA Completion Portfolio (the Fund) seeks long-term capital appreciation. The Fund is intended to be used as part of a broader separately managed account (SMA) program. The objective of the Fund is intended to be evaluated in the context of the broader SMA program. The Fund is not designed to be used as a stand-alone investment.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. The Fund does not pay a management fee to the Fund’s investment manager, Columbia Management Investment Advisers, LLC (the Investment Manager). However, Fund shares may only be purchased and held by or on behalf of SMAs where the Investment Manager has an agreement with the SMA program sponsor (the Program Sponsor), or directly with the SMA client, to provide investment management services to the Program Sponsor or the SMA. SMAs pay a fee directly, or indirectly through Program Sponsors, to the Investment Manager for providing investment management services to the Program Sponsor or the SMA, including on assets that may be invested in the Fund. You should read carefully the SMA program information provided to you by your Program Sponsor or financial intermediary, including information about the fees charged to you and the fees paid to them for SMA investment management services.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	December 31, 2022
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund may pay 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 fund operating expenses or in the example, affect the Fund’s performance. Because the Fund is newly organized, portfolio turnover information is not available as of the date of this prospectus.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	Other expenses are based on estimated amounts for the Fund’s current fiscal year.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that: you invest $10,000 in the Fund for the periods indicated, your investment has a 5% return each year, and the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above. Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are reflected in both examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs (based on estimated Fund expenses) would be:
Expense Example Closing [Text Block]	rr_ExpenseExampleClosingTextBlock	SMA clients pay a fee directly, or indirectly through Program Sponsors, to the Investment Manager for providing investment management services to the Program Sponsor or the SMA, including on assets that may be invested in the Fund. Such fees are not reflected in the Example.
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 80% of total assets (including the amount of any borrowings for investment purposes) in equity securities of foreign companies that have market capitalizations of more than $1 billion at the time of purchase that the Investment Manager believes are undervalued. Foreign companies are those economically tied to a non-U.S. country. The Fund is intended to be used as part of a broader SMA program, and it is expected that the Fund will hold securities that the Investment Manager believes will help the overall SMA program achieve its investment objective, but that the Investment Manager believes may be transacted in more efficiently through a collective vehicle, such as the Fund, than by each individual SMA. The Investment Manager currently expects that these investments will consist primarily of securities listed only on foreign exchanges (referred to as foreign ordinary shares) and relatively small positions in American depositary receipts. The performance and objectives of the Fund should be evaluated in the context of the broader SMA program. The Fund is not designed to be used as a stand-alone investment. The Fund typically invests in foreign companies in at least three countries, other than the United States, at any one time and may invest in emerging markets. The Fund may invest directly in foreign securities, including common and preferred stocks, or indirectly through depositary receipts. From time to time, the Fund may focus its investments in certain countries or geographic areas, including the Asia/Pacific region and Europe. The Fund may from time to time emphasize one or more sectors in selecting its investments, including the financial services sector. The Fund is non-diversified, which means that it can invest a greater percentage of its assets in the securities of fewer issuers than can a diversified fund. The Fund expects to invest in a limited or small number of holdings relative to a traditional mutual fund that is offered more broadly to non-SMA accounts.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the Fund involves risks, including Completion Funds Risk, Non-Diversified Fund Risk, Focused Portfolio Risk, Foreign Securities Risk, Market Risk, Issuer Risk, Value Securities Risk, and Depositary Receipts Risk. The Fund’s Focused Portfolio Risk heightens the Fund’s other risks due to the Fund’s investment in a small number of holdings, which consist primarily of securities listed only on foreign exchanges. Descriptions of these and other principal risks of investing in the Fund are provided below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Active Management Risk. Due to its active management, the Fund could underperform its benchmark index and/or other funds with similar investment objectives and/or strategies. Completion Funds Risk. Investors should be aware that the investments made by the Fund and the results achieved by the Fund at any given time are not expected to be the same as those made by other funds for which the Investment Manager serves as investment adviser, including funds with names, investment objectives and policies similar to the Fund. This may be attributable to a wide variety of factors, including, but not limited to, the use of a differentiated investment strategy. The Fund is intended to be used as part of a broader SMA program. The performance and objectives of the Fund should be evaluated in the context of the broader SMA program. The Fund is not designed to be used as a stand-alone investment. Please contact your program sponsor or financial intermediary for more information. Depositary Receipts Risk. Depositary receipts are receipts issued by a bank or trust company reflecting ownership of underlying securities issued by foreign companies. Some foreign securities are traded in the form of American Depositary Receipts and/or Global Depositary Receipts. Depositary receipts involve risks similar to the risks associated with investments in foreign securities, including those associated with investing in the particular country of an issuer, which may be related to the particular political, regulatory, economic, social and other conditions or events, including, for example, military confrontations, war and terrorism, occurring in the country and fluctuations in such country’s currency, as well as market risk tied to the underlying foreign company. In addition, holders of depositary receipts may have limited voting rights, may not have the same rights afforded to stockholders of a typical domestic company in the event of a corporate action, such as an acquisition, merger or rights offering, and may experience difficulty in receiving company stockholder communications. There is no guarantee that a financial institution will continue to sponsor a depositary receipt, or that a depositary receipt will continue to trade on an exchange, either of which could adversely affect the liquidity, availability and pricing of the depositary receipt. Changes in foreign currency exchange rates will affect the value of depositary receipts and, therefore, may affect the value of your investment in the Fund. Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations. Focused Portfolio Risk. Because the Fund may invest in a limited number of companies, the Fund as a whole is subject to greater risk of loss if any of those securities decline in price. Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities. The performance of the Fund may also be negatively affected by fluctuations in a foreign currency's strength or weakness relative to the U.S. dollar, particularly to the extent the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar. Geographic Focus Risk. The Fund may be particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries within the specific geographic regions in which the Fund invests. The Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. Asia Pacific Region. Many of the countries in the Asia Pacific region are considered underdeveloped or developing, including from a political, economic and/or social perspective, and may have relatively unstable governments and economies based on limited business, industries and/or natural resources or commodities. Events in any one country within the region may impact other countries in the region or the region as a whole. As a result, events in the region will generally have a greater effect on the Fund than if the Fund were more geographically diversified. This could result in increased volatility in the value of the Fund’s investments and losses for the Fund. Also, securities of some companies in the region can be less liquid than U.S. or other foreign securities, potentially making it difficult for the Fund to sell such securities at a desirable time and price. Europe. The Fund is particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries in Europe. In addition, the private and public sectors’ debt problems of a single European Union (EU) country can pose significant economic risks to the EU as a whole. As a result, the Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. If securities of issuers in Europe fall out of favor, it may cause the Fund to underperform other funds that do not focus their investments in this region of the world. The impact of any partial or complete dissolution of the EU on the United Kingdom (UK) and European economies and the broader global economy could be significant, resulting in negative impacts on currency and financial markets generally, such as increased volatility and illiquidity, and potentially lower economic growth in markets in Europe, which may adversely affect the value of your investment in the Fund. Issuer Risk. An issuer in which the Fund invests or to which it has exposure may perform poorly, and the value of its securities may therefore decline, which may negatively affect the Fund’s performance. Underperformance of an issuer may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies. Investments in larger, more established companies may involve certain risks associated with their larger size. For instance, larger, more established companies may be less able to respond quickly to new competitive challenges, such as changes in consumer tastes or innovation from smaller competitors. Also, larger companies are sometimes less able to attain the high growth rates of successful smaller companies, especially during extended periods of economic expansion. Liquidity Risk. Liquidity risk is the risk associated with any event, circumstance, or characteristic of an investment or market that negatively impacts the Fund’s ability to sell, or realize the proceeds from the sale of, an investment at a desirable time or price. Liquidity risk may arise because of, for example, a lack of marketability of the investment, which means that when seeking to sell its portfolio investments, the Fund could find that selling is more difficult than anticipated, especially during times of high market volatility. Market participants attempting to sell the same or a similar instrument at the same time as the Fund could exacerbate the Fund’s exposure to liquidity risk. The Fund may have to accept a lower selling price for the holding, sell other liquid or more liquid investments that it might otherwise prefer to hold (thereby increasing the proportion of the Fund’s investments in less liquid or illiquid securities), or forego another more appealing investment opportunity. Certain investments that were liquid when purchased by the Fund may later become illiquid, particularly in times of overall economic distress. Changing regulatory, market or other conditions or environments (for example, the interest rate or credit environments) may also adversely affect the liquidity and the price of the Fund's investments. Judgment plays a larger role in valuing illiquid or less liquid investments as compared to valuing liquid or more liquid investments. Price volatility may be higher for illiquid or less liquid investments as a result of, for example, the relatively less frequent pricing of such securities (as compared to liquid or more liquid investments). Generally, the less liquid the market at the time the Fund sells a portfolio investment, the greater the risk of loss or decline of value to the Fund. Overall market liquidity and other factors can lead to an increase in redemptions, which may negatively impact Fund performance and NAV, including, for example, if the Fund is forced to sell investments in a down market. Foreign securities can present enhanced liquidity risks, including as a result of less developed custody, settlement or other practices of foreign markets. Market Risk. The market values of securities or other investments that the Fund holds may fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods. New Fund Risk. Investors in newly formed funds bear the risk that the fund may not be successful in implementing its investment strategy, and may not employ a successful investment strategy, either of which could result in the fund being liquidated at any time without shareholder approval and/or at a time that may not be favorable for certain shareholders. Such a liquidation could have negative tax consequences for shareholders. Non-Diversified Fund Risk. The Fund is non-diversified, which generally means that it will invest a greater percentage of its total assets in the securities of fewer issuers than a “diversified” fund. This increases the risk that a change in the value of any one investment held by the Fund could affect the overall value of the Fund more than it would affect that of a diversified fund holding a greater number of investments. Accordingly, the Fund's value will likely be more volatile than the value of a more diversified fund. Preferred Stock Risk. Preferred stock is a type of stock that generally pays dividends at a specified rate and that has preference over common stock in the payment of dividends and the liquidation of assets. Preferred stock does not ordinarily carry voting rights. The price of a preferred stock is generally determined by earnings, type of products or services, projected growth rates, experience of management, liquidity, and general market conditions of the markets on which the stock trades. The most significant risks associated with investments in preferred stock include issuer risk, market risk and interest rate risk (i.e., the risk of losses attributable to changes in interest rates). Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business within one or more economic sectors, including the financial services sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more broadly the Fund invests, the more it spreads risk and potentially reduces the risks of loss and volatility. Financial Services Sector. The Fund may be more susceptible to the particular risks that may affect companies in the financial services sector than if it were invested in a wider variety of companies in unrelated sectors. Companies in the financial services sector are subject to certain risks, including the risk of regulatory change, decreased liquidity in credit markets and unstable interest rates. Such companies may have concentrated portfolios, such as a high level of loans to one or more industries or sectors, which makes them vulnerable to economic conditions that affect such industries or sectors. Performance of such companies may be affected by competitive pressures and exposure to investments or agreements that, under certain circumstances, may lead to losses (e.g., subprime loans). Companies in the financial services sector are subject to extensive governmental regulation that may limit the amount and types of loans and other financial commitments they can make, and the interest rates and fees they may charge. In addition, profitability of such companies is largely dependent upon the availability and the cost of capital. Value Securities Risk. Value securities are securities of companies that may have experienced, for example, adverse business, industry or other developments or may be subject to special risks that have caused the securities to be out of favor and, in turn, potentially undervalued. The market value of a portfolio security may not meet portfolio management’s perceived value assessment of that security, or may decline in price, even though portfolio management believes the securities are already undervalued. There is also a risk that it may take longer than expected for the value of these investments to rise to portfolio management’s perceived value. In addition, value securities, at times, may not perform as well as growth securities or the stock market in general, and may be out of favor with investors for varying periods of time.
Risk Lose Money [Text]	rr_RiskLoseMoney	There is no assurance that the Fund will achieve its investment objective and you may lose money.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Nondiversified Status [Text]	rr_RiskNondiversifiedStatus	Non-Diversified Fund Risk. The Fund is non-diversified, which generally means that it will invest a greater percentage of its total assets in the securities of fewer issuers than a “diversified” fund. This increases the risk that a change in the value of any one investment held by the Fund could affect the overall value of the Fund more than it would affect that of a diversified fund holding a greater number of investments. Accordingly, the Fund's value will likely be more volatile than the value of a more diversified fund.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The Fund is new as of the date of this prospectus and therefore performance information is not available. When available, the Fund intends to compare its performance to the performance of MSCI EAFE Value Index (Net); however, the benchmark is not intended to represent how the Fund is expected to perform. When available, updated performance information can be obtained by calling toll-free 800.345.6611 or visiting https://www.columbiathreadneedleus.com/investment-products/managed-accounts/. The Fund is intended to be used as part of a broader SMA program. The performance and objectives of the Fund should be evaluated in the context of the broader SMA program. The Fund is only offered to SMA clients as described in this Prospectus, and the Fund’s performance, when available, will not reflect payment to Program Sponsors or the Investment Manager of any applicable fees by clients in SMA programs and will differ from the performance of a participant’s overall SMA. For more information about your SMA’s performance, please contact your Program Sponsor or financial intermediary.
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	The Fund is new as of the date of this prospectus and therefore performance information is not available.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.345.6611
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	https://www.columbiathreadneedleus.com/investment-products/managed-accounts/
Overseas SMA Completion Portfolio | Overseas SMA Completion Portfolio
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	none	[1]
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	6.44%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	6.44%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(6.44%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	none
1 year	rr_ExpenseExampleYear01	none
3 years	rr_ExpenseExampleYear03	none
1 year	rr_ExpenseExampleNoRedemptionYear01	none
3 years	rr_ExpenseExampleNoRedemptionYear03	none

[1] The Investment Manager of the Fund does not charge a management fee to the Fund. However, the Fund is an integral part of broader SMA programs, and SMA clients pay a fee directly, or indirectly through Program Sponsors, to the Investment Manager for providing investment management services to the Program Sponsor or the SMA, including on assets that may be invested in the Fund. The Investment Manager or its affiliates will be compensated directly or indirectly by Program Sponsors or SMAs for providing investment management services to the SMA.

[2] Other expenses are based on estimated amounts for the Fund’s current fiscal year.

[3] Columbia Management Investment Advisers, LLC and certain of its affiliates have contractually agreed to waive fees and/or to reimburse expenses (excluding transaction costs and certain other investment related expenses, interest, taxes, and infrequent and/or unusual expenses) through December 31, 2022, unless sooner terminated at the sole discretion of the Fund’s Board of Trustees. Under this agreement, the Fund’s net operating expenses, subject to applicable exclusions, will not exceed the annual rate of 0.00%.