0000701219 ccr1:S000005142Member 2012-02-01 2013-01-31 0000701219 2012-02-01 2013-01-31 0000701219 ccr1:S000005142Member ccr1:C000014076Member 2012-02-01 2013-01-31 0000701219 ccr1:S000005142Member ccr1:LipperInstitutionalMoneyMarketFundsavgMember 2012-02-01 2013-01-31 pure iso4217:USD <div style="display:none">~ http://www.calvert.com/role/ScheduleShareholderFeesInstitutionalPrimeFund column period compact * ~</div> <div style="display:none">~ http://www.calvert.com/role/ScheduleAnnualFundOperatingExpensesInstitutionalPrimeFund column period compact * ~</div> <div style="display:none">~ http://www.calvert.com/role/ScheduleExpenseExampleTransposedInstitutionalPrimeFund column period compact * ~</div> <div style="display:none">~ http://www.calvert.com/role/ScheduleAnnualTotalReturnsInstitutionalPrimeFundBarChart column period compact * ~</div> 2013-01-31 485BPOS CALVERT CASH RESERVES 0000701219 false 2013-01-30 2013-01-31 2012-09-30 INVESTMENT OBJECTIVE The Fund seeks to obtain the highest level of current income, consistent with safety, preservation of capital and liquidity that is available through investments in specified money market instruments. The Fund seeks to maintain a constant net asset value of $1.00 per share. This objective may be changed by the Fund's Board of Trustees without shareholder approval. FEES AND EXPENSES OF THE FUND This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. Shareholder Fees (fees paid directly from your investment) 0 0 Annual Fund Operating Expenses (expenses that you pay each year as a % of the value of your investment) 0.003 0.0011 0 0.0041 -0.0001 0.004 Example 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:<ul type="square"><li>you invest $1,000,000 in the Fund for the time periods indicated;</li><li> your investment has a 5% return each year;</li><li>the Fund’s operating expenses remain the same; and</li><li>any Calvert expense limitation is in effect for the period indicated in the fee table above.</li></ul>Although your actual costs may be higher or lower, under these assumptions your costs would be: 4092 13068 22889 51679 INVESTMENTS, RISKS AND PERFORMANCE Principal Investment Strategies The Fund’s assets are invested primarily in top-tier securities, such as:<ul type="square"><li>high-quality, short-term investments, including obligations of the U.S. Government and agency or instrumentality securities;</li></ul><ul type="square"><li>high-quality, U.S. dollar-denominated international money market investments;</li></ul><ul type="square"><li>certificates of deposit of major banks;</li></ul><ul type="square"><li>commercial paper;</li></ul><ul type="square"><li>eligible high-grade, short-term corporate obligations and participation interests in such obligations;</li></ul><ul type="square"><li>repurchase agreements;</li></ul><ul type="square"><li>bankers acceptances;</li></ul><ul type="square"><li>floating rate notes;</li></ul><ul type="square"><li>variable-rate demand notes; and</li></ul><ul type="square"><li>taxable municipal securities.</li></ul>The Fund invests in accordance with Rule 2a-7 under the Investment Company Act of 1940, as amended. Principal Risks You could lose money on your investment in the Fund, or the Fund could underperform, because of the risks described below. 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. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the Fund. Income Risk. The income level of the Fund will fluctuate with changing market conditions and interest rate levels. The income the Fund receives may fall as a result of a decline in interest rates. Interest Rate Risk. A change in interest rates may adversely affect the value of fixed-income securities. When interest rates rise, the value of fixed-income securities will generally fall. Longer-term securities are subject to greater interest rate risk. Credit Risk. The credit quality of fixed-income securities may deteriorate, which could lead to default or bankruptcy of the issuer where the issuer becomes unable to pay its obligations when due. Credit risk, however, should be low for the Fund because it invests primarily in securities that are considered to be of high quality. The Fund also limits the amount it invests in any one issuer to try to lessen its exposure to credit risk. Management Risk. The individual investments of the Fund may not perform as well as expected, due to credit, political or other risks and/or the Fund’s portfolio management practices may not achieve the desired result. Repurchase Agreement Risk. A repurchase agreement exposes the Fund to the risk that the party that sells the security may default on its obligation to repurchase it. The Fund may lose money because it cannot sell the security at the agreed-upon time and price or the security loses value before it can be sold. Performance The following bar chart and table show the Fund’s annual returns and its long-term performance, which give some indication of the risks of investing in the Fund. The bar chart shows how the performance of the Fund’s shares has varied from year to year. The table compares the Fund’s performance over time with that of an average. The Fund’s past performance (before and after taxes) does not necessarily indicate how the Fund will perform in the future. For updated performance information, visit www.calvert.com. 0.0108 0.0126 0.0313 0.0487 0.0511 0.0307 0.0066 0.001 0.0001 0.0001 Average Annual Total Returns (as of 12-31-12) 0.0001 0.0006 0.0076 0.0061 0.0191 0.0175                                                                                                      Quarter                           Total                                                                                                        Ended                           Return          Best Quarter (of periods shown)                                                       6/30/07                          1.28% Worst Quarter (of periods shown)                                                  12/31/11                          0.00% For current yield information, call 800-317-2274, or visit Calvert’s website at www.calvert.com/institutional. Best Quarter (of periods shown) 2007-06-30 0.0128 Worst Quarter (of periods shown) 2011-12-31 0 January 31, 2014 You could lose money on your investment in the Fund, or the Fund could underperform, because of the risks described below. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the Fund. 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. The bar chart shows how the performance of the Fund’s shares has varied from year to year. www.calvert.com The Fund’s past performance (before and after taxes) does not necessarily indicate how the Fund will perform in the future. <div style="display:none">~ http://www.calvert.com/role/ScheduleAverageAnnualTotalReturnsTransposedInstitutionalPrimeFund column period compact * ~</div> CALVERT CASH RESERVES (“CCR”) INSTITUTIONAL PRIME FUND Class (Ticker):     CCIXX Calendar Year Total Return 800-317-2274 The investment advisor has agreed to contractually limit direct net annual fund operating expenses through January 31, 2014. Direct net operating expenses will not exceed 0.40% for Class I. Only the Board of Trustees of the Fund may terminate the Fund's expense limitation before the contractual period expires.