Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	CASH ACCOUNT TRUST
Prospectus Date	rr_ProspectusDate	Aug. 01, 2012
Service Shares | Tax-Exempt Portfolio
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	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	The fund normally invests at least 80% of its net assets 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 managed in accordance with Rule 2a-7 under the Investment Company Act of 1940, as amended, which governs the quality, maturity, diversity and liquidity of instruments in which a money fund may invest. The fund may invest in municipal trust securities (MTRs), general obligation and revenue notes and bonds, municipal obligations backed by third parties 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-free income along with the liquidity and stability that a money fund is designed to offer. 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. An investment in the fund is not insured or guaranteed by the FDIC or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, this share price isn't guaranteed, and if it falls below $1.00 you would lose money. The Advisor is not obligated to take any action to maintain the $1.00 share price. The share price could fall below $1.00 as a result of the actions of one or more large investors in the fund. The credit quality of the fund's holdings can change rapidly in certain markets, and the default of a single holding could cause the fund's share price to fall below $1.00, as could periods of high redemption pressures and/or illiquid markets. 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. 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. Credit risk. The fund's performance could be hurt if an issuer of a money market instrument suffers an adverse change in financial condition that results in a payment default, security downgrade or inability to meet a financial obligation. Municipal securities risk. The fund will be impacted by events in the municipal securities market. Negative events, such as severe fiscal difficulties, 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 mutual 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 bonds. 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 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. 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.
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. An investment in the fund is not insured or guaranteed by the FDIC or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, this share price isn't guaranteed, and if it falls below $1.00 you would lose money. The Advisor is not obligated to take any action to maintain the $1.00 share price. The share price could fall below $1.00 as a result of the actions of one or more large investors in the fund. The credit quality of the fund's holdings can change rapidly in certain markets, and the default of a single holding could cause the fund's share price to fall below $1.00, as could periods of high redemption pressures and/or illiquid markets.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not insured or guaranteed by the FDIC 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 www.dws-investments.com (the Web site does not form a part of this prospectus) or call the phone number for your share class included in this prospectus. Performance prior to class inception is based on the performance of the fund's DWS Tax-Exempt Cash Institutional Shares, adjusted to reflect the higher expenses of Service Shares. DWS Tax-Exempt Cash Institutional Shares are offered in a separate 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	www.dws-investments.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 (%) (Service Shares)
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	Best Quarter: 0.70%, Q2 2007 Worst Quarter: 0.00%, Q3 2003 Year-to-Date as of 6/30/12: 0.00%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns (for periods ended 12/31/2011 expressed as a %)
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	Total returns would have been lower if operating expenses hadn't been reduced. For more recent performance information, contact the financial services firm from which you obtained this prospectus.
Service Shares | Tax-Exempt Portfolio | Service Shares
Risk/Return:	rr_RiskReturnAbstract
SHAREHOLDER FEES (paid directly from your investment)	rr_ShareholderFeeOther	none
Management fee	rr_ManagementFeesOverAssets	0.06%
Distribution (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.60%
Other expenses	rr_OtherExpensesOverAssets	0.38%
Total annual fund operating expenses	rr_ExpensesOverAssets	1.04%
1 Year	rr_ExpenseExampleYear01	106
3 Years	rr_ExpenseExampleYear03	331
5 Years	rr_ExpenseExampleYear05	574
10 Years	rr_ExpenseExampleYear10	1,271
2002	rr_AnnualReturn2002	0.45%
2003	rr_AnnualReturn2003	0.10%
2004	rr_AnnualReturn2004	0.26%
2005	rr_AnnualReturn2005	1.44%
2006	rr_AnnualReturn2006	2.47%
2007	rr_AnnualReturn2007	2.70%
2008	rr_AnnualReturn2008	1.38%
2009	rr_AnnualReturn2009	0.02%
2010	rr_AnnualReturn2010	0.01%
2011	rr_AnnualReturn2011	0.02%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year-to-Date
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Jun. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.00%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2007
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	0.70%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2003
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.00%
Class Inception	rr_AverageAnnualReturnInceptionDate	May 13, 2005
1 Year	rr_AverageAnnualReturnYear01	0.02%
5 Years	rr_AverageAnnualReturnYear05	0.82%
10 Years	rr_AverageAnnualReturnYear10	0.88%