Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY CHARLES STREET TRUST
09.30 Fidelity Health Savings & Health Savings Index Funds Retail Combo PRO-02 | Fidelity Health Savings Index Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Summary Fund:Fidelity® Health Savings Index Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks total return.
Expense [Heading]	rr_ExpenseHeading	Fee Table
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees
(fees paid directly from your investment)	rr_ShareholderFeeOther	none
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Operating Expenses (expenses that you pay each year as a % of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	January 31, 2022
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	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. For the period from March 2, 2020 to September 30, 2020, the fund's portfolio turnover rate was 71% (annualized) of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	71.00%
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	Differs from the ratios of expenses to average net assets in the Financial Highlights section of the prospectus because of acquired fund fees and expenses.
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	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 your shareholder 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:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The fund is designed to assist investors in saving for future eligible medical expenses. Because the time horizon of such expenses is inherently uncertain, the fund pursues an asset allocation strategy that is designed to balance growth and downside market protection through different market environments. Normally investing at least 80% of assets in a combination of passively managed Fidelity bond and equity funds and affiliated and unaffiliated exchange-traded funds (collectively, underlying funds). Investing approximately 30% of the fund’s assets in underlying equity funds and 70% of the fund’s assets in underlying bond funds. The Adviser may vary the fund's exposure to the underlying funds within the following ranges: 20-40% of the fund's assets in underlying equity funds and 60-80% of the fund's assets in underlying bond funds. Allocating the fund’s assets among bond funds (including domestic, international and emerging markets, U.S. inflation-protected debt, and U.S. long-term treasuries) and equity funds (including domestic, international, and emerging markets equities) to manage the fund’s risk across asset classes over time. Investing up to 25% of the fund’s assets in underlying high income (including funds that invest in high yield debt securities (also referred to as junk bonds)), leveraged loan, country specific equity, real estate equity and commodities funds. Investing in Fidelity's central funds (specialized investment vehicles used by Fidelity® funds to invest in particular security types or investment disciplines) consistent with the asset classes discussed above. Buying and selling futures contracts (both long and short positions) in an effort to manage cash flows efficiently, remain fully invested, or facilitate asset allocation.
Risk [Heading]	rr_RiskHeading	Principal Investment Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Asset Allocation Risk. The fund is subject to risks resulting from the Adviser's asset allocation decisions. The selection of underlying funds and the allocation of the fund's assets among various asset classes could cause the fund to lose value or its results to lag relevant benchmarks or other funds with similar objectives. Investing in Other Funds. The fund bears all risks of investment strategies employed by the underlying funds, including the risk that the underlying funds will not meet their investment objectives. Stock Market Volatility. Stock markets are volatile and can decline significantly in response to adverse issuer, political, regulatory, market, or economic developments. Different parts of the market, including different market sectors, and different types of securities can react differently to these developments. Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease. Foreign Exposure. Foreign markets, particularly emerging markets, can be more volatile than the U.S. market due to increased risks of adverse issuer, political, regulatory, market, or economic developments and can perform differently from the U.S. market. Emerging markets can be subject to greater social, economic, regulatory, and political uncertainties and can be extremely volatile. Foreign exchange rates also can be extremely volatile. Foreign Currency Transactions. Although a forward foreign currency exchange contract is used to reduce or hedge a fund's exposure to changes in the value of the currency, suitable hedging transactions may not be available in all circumstances, may not be successful, and may eliminate any chance for the fund to benefit from favorable fluctuations in relevant foreign currencies. Industry Exposure. Market conditions, interest rates, and economic, regulatory, or financial developments could significantly affect a single industry or group of related industries. Prepayment. The ability of an issuer of a debt security to repay principal prior to a security's maturity can cause greater price volatility if interest rates change. Issuer-Specific Changes. The value of an individual security or particular type of security can be more volatile than, and can perform differently from, the market as a whole. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease. Lower-quality debt securities (those of less than investment-grade quality, also referred to as high yield debt securities or junk bonds) and certain types of other securities involve greater risk of default or price changes due to changes in the credit quality of the issuer. The value of lower-quality debt securities and certain types of other securities can be more volatile due to increased sensitivity to adverse issuer, political, regulatory, market, or economic developments. Correlation to Index. The performance of an underlying index fund and its index may vary somewhat due to factors such as fees and expenses of the underlying fund, transaction costs, sample selection, regulatory restrictions, and timing differences associated with additions to and deletions from the index. Errors in the construction or calculation of the index may occur from time to time and may not be identified and corrected for some period of time, which may have an adverse impact on an underlying fund and its shareholders. Passive Management Risk. Some of the underlying funds in which the fund invests are managed with a passive investment strategy, attempting to track the performance of an unmanaged index of securities, regardless of the current or projected performance of an underlying fund's index or of the actual securities included in the index. This differs from an actively managed fund, which typically seeks to outperform a benchmark index. As a result, the performance of these underlying funds could be lower than actively managed funds that may shift their portfolio assets to take advantage of market opportunities or lessen the impact of a market decline or a decline in the value of one or more issuers. An underlying index fund may be concentrated to approximately the same extent that its index concentrates in the securities of issuers in a particular industry or group of industries. Leverage Risk. Leverage can increase market exposure, magnify investment risks, and cause losses to be realized more quickly. Investing in ETFs. ETFs may trade in the secondary market at prices below the value of their underlying portfolios and may not be liquid. ETFs that track an index are subject to tracking error and may be unable to sell poorly performing assets that are included in their index or other benchmark. Investing for Inflation Protection. Increases in real interest rates can cause the price of inflation-protected debt securities to decrease. Interest payments on inflation-protected debt securities can be unpredictable. Securities Lending Risk. Securities lending involves the risk that the borrower may fail to return the securities loaned in a timely manner or at all. If the borrower defaults on its obligation to return the securities loaned because of insolvency or other reasons, an underlying fund could experience delays and costs in recovering the securities loaned or in gaining access to the collateral. An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. You could lose money by investing in the fund.
Risk Lose Money [Text]	rr_RiskLoseMoney	You could lose money by investing in the fund.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	Performance history will be available for the fund after the fund has been in operation for one calendar year.
09.30 Fidelity Health Savings & Health Savings Index Funds Retail Combo PRO-02 | Fidelity Health Savings Index Fund | Fidelity Health Savings Index Fund
Risk/Return:	rr_RiskReturnAbstract
Management fee	rr_ManagementFeesOverAssets	0.15%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	none
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.09%
Total annual operating expenses	rr_ExpensesOverAssets	0.24%	[1]
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	0.05%	[2]
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	0.19%	[1]
1 year	rr_ExpenseExampleYear01	$ 19
3 years	rr_ExpenseExampleYear03	70
5 years	rr_ExpenseExampleYear05	128
10 years	rr_ExpenseExampleYear10	$ 299

[1] ^(a)Differs from the ratios of expenses to average net assets in the Financial Highlights section of the prospectus because of acquired fund fees and expenses.

[2] ^(b)Fidelity Management & Research Company LLC (FMR) has contractually agreed to waive 0.05% of the fund's management fee. This arrangement will remain in effect through January 31, 2022. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.