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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName CASH ACCOUNT TRUST
Prospectus Date rr_ProspectusDate Aug. 01, 2018
Tax Free Investment Class | DWS Tax-Exempt Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading

DWS Tax-Exempt Portfolio

(formerly Deutsche Tax-Exempt Portfolio)
Objective [Heading] rr_ObjectiveHeading

Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund seeks to provide maximum current income that is exempt from federal income taxes to the extent consistent with stability of capital.
Expense [Heading] rr_ExpenseHeading

Fees and Expenses of the Fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

These are the fees and expenses you may pay when you buy and hold shares.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption SHAREHOLDER FEES (paid directly from your investment)
Operating Expenses Caption [Text] rr_OperatingExpensesCaption ANNUAL FUND OPERATING EXPENSES
(expenses that you pay each year as a % of the value of your investment)
Expense Example [Heading] rr_ExpenseExampleHeading EXAMPLE
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This 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 assumes that you invest $10,000 in the fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the fund's operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading

Principal Investment Strategy
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

Main investments. The fund normally invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in municipal securities, the income from which is free from regular federal income tax and alternative minimum tax (AMT). This policy is fundamental and may not be changed without shareholder approval.

The fund is a money market fund that is managed in accordance with federal regulations, which govern the quality, maturity, diversity and liquidity of instruments in which a money market fund may invest.

The fund follows policies designed to maintain a stable $1.00 share price.

The fund may invest in municipal trust receipts (MTRs), general obligation and revenue notes and bonds, municipal obligations backed by third parties, obligations of the territories or Commonwealths of the US and other municipal instruments paying a fixed, variable or floating interest rate.

The fund is designed for investors in a moderate to high tax bracket who are interested in federal tax-exempt income along with the liquidity and stability that a money market fund is designed to offer.

Management process. Working in consultation with portfolio management, a credit team screens potential securities and develops a list of those that the fund may buy. Portfolio management, looking for attractive yield and weighing considerations such as credit quality, economic outlooks and possible interest rate movements, then decides which securities on this list to buy.
Risk [Heading] rr_RiskHeading

Main Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

There are several risk factors that could reduce the yield you get from the fund, cause the fund's performance to trail that of other investments, or cause you to lose money.

Money market fund risk. You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The fund's sponsor has no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Interest rate risk. Rising interest rates could cause the value of the fund's investments — and therefore its share price as well — to decline. Conversely, any decline in interest rates is likely to cause the fund's yield to decline, and during periods of unusually low interest rates, the fund's yield may approach zero. A low interest rate environment may prevent the fund from providing a positive yield or paying fund expenses out of current income and, at times, could impair the fund's ability to maintain a stable $1.00 share price. Over time, the total return of a money market fund may not keep pace with inflation, which could result in a net loss of purchasing power for long-term investors. Recent and potential future changes in monetary policy made by central banks and/or their governments are likely to affect the level of interest rates. Money market funds try to minimize this risk by purchasing short-term securities.

Credit risk. The fund's performance could be hurt and the fund's share price could fall below $1.00 if an issuer of a debt security suffers an adverse change in financial condition that results in the issuer not making timely payments of interest or principal, a security downgrade or an inability to meet a financial obligation.

Liquidity and transaction risk. The liquidity of portfolio securities can deteriorate rapidly due to credit events affecting issuers or guarantors or due to general market conditions and a lack of willing buyers. When there are no willing buyers and an instrument cannot be readily sold at a desired time or price, the fund may have to accept a lower price or may not be able to sell the instrument at all. If dealer capacity in debt instruments is insufficient for market conditions, it may further inhibit liquidity and increase volatility in the debt markets. Additionally, market participants other than the fund may attempt to sell debt holdings at the same time as the fund, which could cause downward pricing pressure and contribute to illiquidity. An inability to sell one or more portfolio securities can adversely affect the fund's ability to maintain a $1.00 share price or prevent the fund from being able to take advantage of other investment opportunities.

Unusual market conditions, an unusually high volume of redemption requests or other similar conditions could cause the fund to be unable to pay redemption proceeds within a short period of time. If the fund is forced to sell securities at an unfavorable time and/or under unfavorable conditions, such sales may adversely affect the fund's ability to maintain a $1.00 share price.

Municipal securities risk. The fund could be impacted by events in the municipal securities market, including the supply and demand for municipal securities. Negative events, such as severe fiscal difficulties, bankruptcy of one or more issuers, an economic downturn, unfavorable legislation, court rulings or political developments, or reduced monetary support from the federal government could hurt fund performance.

Security selection risk. Although short-term securities are relatively stable investments, it is possible that the securities in which the fund invests will not perform as expected. This could cause the fund's returns to lag behind those of similar money market funds and could result in a decline in share price.

Municipal trust receipts risk. The fund's investment in MTRs is subject to similar risks as other investments in debt obligations, including interest rate risk, credit risk and security selection risk. Additionally, investments in MTRs raise certain tax issues that may not be presented by direct investments in municipal securities. There is some risk that certain issues could be resolved in a manner that could adversely impact the performance of the fund.

Tax risk. Any distributions to shareholders that represent income from taxable securities will generally be taxable as ordinary income at both the state and federal levels, while other distributions, such as capital gains, are taxable to the same extent they would be for any mutual fund. New federal or state governmental action could adversely affect the tax-exempt status of securities held by the fund, resulting in a higher tax liability for shareholders and potentially hurting fund performance as well.

Counterparty risk. A financial institution or other counterparty with whom the fund does business, or that underwrites, distributes or guarantees any investments or contracts that the fund owns or is otherwise exposed to, may decline in financial health and become unable to honor its commitments. This could cause losses for the fund or could delay the return or delivery of collateral or other assets to the fund.

US territory and Commonwealth obligations risk. Adverse political and economic conditions and developments affecting any territory or Commonwealth of the US may, in turn, negatively affect the value of the fund's holdings in such obligations.

Prepayment and extension risk. When interest rates fall, issuers of high interest debt obligations may pay off the debts earlier than expected (prepayment risk), and the fund may have to reinvest the proceeds at lower yields. When interest rates rise, issuers of lower interest debt obligations may pay off the debts later than expected (extension risk), thus keeping the fund's assets tied up in lower interest debt obligations. Ultimately, any unexpected behavior in interest rates could increase the volatility of the fund's yield and could hurt fund performance. Prepayments could also create capital gains tax liability in some instances.

Risks of holding cash. The fund will at times hold cash positions, which may hurt the fund's performance. Cash positions may also subject the fund to additional risks and costs, including any fees imposed by the fund's custodian for large cash balances.

Fees and gates risk. The fund has adopted policies and procedures such that the fund will be able to impose liquidity fees on redemptions and/or temporarily suspend (gate) redemptions for up to 10 business days in any 90 day period in the event that the fund's liquidity falls below required minimums. A liquidity fee would reduce the amount shareholders receive upon redemption of shares. Redemption gates would prevent shareholders from redeeming fund shares.

Operational and technology risk. Cyber-attacks, disruptions, or failures that affect the fund's service providers or counterparties, issuers of securities held by the fund, or other market participants may adversely affect the fund and its shareholders, including by causing losses for the fund or impairing fund operations.
Risk Lose Money [Text] rr_RiskLoseMoney There are several risk factors that could reduce the yield you get from the fund, cause the fund's performance to trail that of other investments, or cause you to lose money.
Risk Money Market Fund [Text] rr_RiskMoneyMarketFund Money market fund risk. You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The fund's sponsor has no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading

Past Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

How a fund's returns vary from year to year can give an idea of its risk. Past performance may not indicate future results. All performance figures below assume that dividends were reinvested. The 7-day yield, which is often referred to as the "current yield," is the income generated by the fund over a seven-day period. This amount is then annualized, which means that we assume the fund generates the same income every week for a year. For more recent performance figures and the current yield, go to dws.com (the Web site does not form a part of this prospectus) or call the telephone number included in this prospectus.

Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns How a fund's returns vary from year to year can give an idea of its risk.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress dws.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture Past performance may not indicate future results.
Bar Chart [Heading] rr_BarChartHeading CALENDAR YEAR TOTAL RETURNS (%) (Tax-Free Investment Class)
Bar Chart Narrative [Text Block] rr_BarChartNarrativeTextBlock Returns for other classes were different and are not shown here.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Returns Period ending
Best Quarter 0.56% March 31, 2008
Worst Quarter 0.00% March 31, 2010
Year-to-Date 0.38% June 30, 2018

Performance Table Heading rr_PerformanceTableHeading

Average Annual Total Returns
(For periods ended 12/31/2017 expressed as a %) (Tax-Free Investment Class)
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock

Total returns would have been lower if operating expenses had not been reduced.

For more recent performance information, contact the financial services firm from which you obtained this prospectus.
Tax Free Investment Class | DWS Tax-Exempt Portfolio | Tax-Free Investment Class  
Risk/Return: rr_RiskReturnAbstract  
(paid directly from your investment) rr_ShareholderFeeOther none
Management fee rr_ManagementFeesOverAssets 0.07%
Distribution/service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 0.40%
Total annual fund operating expenses rr_ExpensesOverAssets 0.72%
1 Year rr_ExpenseExampleYear01 $ 74
3 Years rr_ExpenseExampleYear03 230
5 Years rr_ExpenseExampleYear05 401
10 Years rr_ExpenseExampleYear10 $ 894
2008 rr_AnnualReturn2008 1.77%
2009 rr_AnnualReturn2009 0.12%
2010 rr_AnnualReturn2010 0.01%
2011 rr_AnnualReturn2011 0.02%
2012 rr_AnnualReturn2012 0.02%
2013 rr_AnnualReturn2013 0.04%
2014 rr_AnnualReturn2014 0.03%
2015 rr_AnnualReturn2015 0.02%
2016 rr_AnnualReturn2016 0.11%
2017 rr_AnnualReturn2017 0.22%
Year to Date Return, Label rr_YearToDateReturnLabel Year-to-Date
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Jun. 30, 2018
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 0.38%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2008
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.56%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2010
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn none
1 Year rr_AverageAnnualReturnYear01 0.22%
5 Years rr_AverageAnnualReturnYear05 0.08%
10 Years rr_AverageAnnualReturnYear10 0.23%
Class Inception rr_AverageAnnualReturnInceptionDate Mar. 19, 2007