<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.19.3.a.u2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>214</ContextCount>
  <ElementCount>365</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>63</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>100000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/StatementCONSOLIDATEDBALANCESHEETS</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100020 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/StatementCONSOLIDATEDBALANCESHEETSParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100030 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/StatementCONSOLIDATEDSTATEMENTSOFOPERATIONS</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF OPERATIONS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100040 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/StatementCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMELOSS</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100050 - Statement - CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/StatementCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100060 - Statement - CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/StatementCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100070 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/StatementCONSOLIDATEDSTATEMENTSOFCASHFLOWS</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>100080 - Disclosure - BUSINESS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureBUSINESS</Role>
      <ShortName>BUSINESS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>100090 - Disclosure - ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureACCOUNTINGPOLICIES</Role>
      <ShortName>ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>100100 - Disclosure - NET INCOME (LOSS) PER COMMON SHARE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureNETINCOMELOSSPERCOMMONSHARE</Role>
      <ShortName>NET INCOME (LOSS) PER COMMON SHARE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>100110 - Disclosure - RESIDENTIAL MORTGAGE INVESTMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureRESIDENTIALMORTGAGEINVESTMENTS</Role>
      <ShortName>RESIDENTIAL MORTGAGE INVESTMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>100120 - Disclosure - SECURED BORROWINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSECUREDBORROWINGS</Role>
      <ShortName>SECURED BORROWINGS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>100130 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENT</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>100140 - Disclosure - UNSECURED BORROWINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUNSECUREDBORROWINGS</Role>
      <ShortName>UNSECURED BORROWINGS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>100150 - Disclosure - FAIR VALUES OF FINANCIAL INSTRUMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureFAIRVALUESOFFINANCIALINSTRUMENTS</Role>
      <ShortName>FAIR VALUES OF FINANCIAL INSTRUMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>100160 - Disclosure - INCOME TAXES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureINCOMETAXES</Role>
      <ShortName>INCOME TAXES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>100170 - Disclosure - STOCKHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSTOCKHOLDERSEQUITY</Role>
      <ShortName>STOCKHOLDERS' EQUITY</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>100180 - Disclosure - EQUITY INCENTIVE PLAN</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureEQUITYINCENTIVEPLAN</Role>
      <ShortName>EQUITY INCENTIVE PLAN</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>100190 - Disclosure - QUARTERLY RESULTS (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureQUARTERLYRESULTSUNAUDITED</Role>
      <ShortName>QUARTERLY RESULTS (UNAUDITED)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>100200 - Disclosure - SUBSEQUENT EVENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSUBSEQUENTEVENTS</Role>
      <ShortName>SUBSEQUENT EVENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>100210 - Disclosure - ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureACCOUNTINGPOLICIESPolicies</Role>
      <ShortName>ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>100220 - Disclosure - NET INCOME (LOSS) PER COMMON SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureNETINCOMELOSSPERCOMMONSHARETables</Role>
      <ShortName>NET INCOME (LOSS) PER COMMON SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureNETINCOMELOSSPERCOMMONSHARE</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>100230 - Disclosure - NET INCOME PER COMMON SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureNETINCOMEPERCOMMONSHARETables</Role>
      <ShortName>NET INCOME PER COMMON SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>100240 - Disclosure - RESIDENTIAL MORTGAGE INVESTMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureRESIDENTIALMORTGAGEINVESTMENTSTables</Role>
      <ShortName>RESIDENTIAL MORTGAGE INVESTMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureRESIDENTIALMORTGAGEINVESTMENTS</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>100250 - Disclosure - SECURED BORROWINGS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSECUREDBORROWINGSTables</Role>
      <ShortName>SECURED BORROWINGS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureSECUREDBORROWINGS</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>100260 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENTTables</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENT</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>100270 - Disclosure - UNSECURED BORROWINGS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUNSECUREDBORROWINGSTables</Role>
      <ShortName>UNSECURED BORROWINGS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureUNSECUREDBORROWINGS</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>100280 - Disclosure - FAIR VALUES OF FINANCIAL INSTRUMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureFAIRVALUESOFFINANCIALINSTRUMENTSTables</Role>
      <ShortName>FAIR VALUES OF FINANCIAL INSTRUMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureFAIRVALUESOFFINANCIALINSTRUMENTS</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>100290 - Disclosure - INCOME TAXES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureINCOMETAXESTables</Role>
      <ShortName>INCOME TAXES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureINCOMETAXES</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>100300 - Disclosure - STOCKHOLDERS' EQUITY (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSTOCKHOLDERSEQUITYTables</Role>
      <ShortName>STOCKHOLDERS' EQUITY (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureSTOCKHOLDERSEQUITY</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>100310 - Disclosure - EQUITY INCENTIVE PLAN (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureEQUITYINCENTIVEPLANTables</Role>
      <ShortName>EQUITY INCENTIVE PLAN (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureEQUITYINCENTIVEPLAN</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>100320 - Disclosure - QUARTERLY RESULTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureQUARTERLYRESULTSTables</Role>
      <ShortName>QUARTERLY RESULTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureQUARTERLYRESULTSUNAUDITED</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>100330 - Disclosure - ACCOUNTING POLICIES - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureACCOUNTINGPOLICIESAdditionalInformationDetails</Role>
      <ShortName>ACCOUNTING POLICIES - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>100340 - Disclosure - NET INCOME (LOSS) PER COMMON SHARE - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureNETINCOMELOSSPERCOMMONSHAREAdditionalInformationDetails</Role>
      <ShortName>NET INCOME (LOSS) PER COMMON SHARE - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureNETINCOMELOSSPERCOMMONSHARETables</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>100350 - Disclosure - NET INCOME (LOSS) PER COMMON SHARE - Components of Computation of Basic and Diluted Net Income per Common Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureNETINCOMELOSSPERCOMMONSHAREComponentsOfComputationOfBasicAndDilutedNetIncomePerCommonShareDetails</Role>
      <ShortName>NET INCOME (LOSS) PER COMMON SHARE - Components of Computation of Basic and Diluted Net Income per Common Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureNETINCOMELOSSPERCOMMONSHARETables</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>100360 - Disclosure - RESIDENTIAL MORTGAGE INVESTMENTS - Schedule of Residential Mortgage Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureRESIDENTIALMORTGAGEINVESTMENTSScheduleOfResidentialMortgageInvestmentsDetails</Role>
      <ShortName>RESIDENTIAL MORTGAGE INVESTMENTS - Schedule of Residential Mortgage Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>100370 - Disclosure - RESIDENTIAL MORTGAGE INVESTMENTS - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureRESIDENTIALMORTGAGEINVESTMENTSAdditionalInformationDetails</Role>
      <ShortName>RESIDENTIAL MORTGAGE INVESTMENTS - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>100380 - Disclosure - SECURED BORROWINGS - Schedule of Secured Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSECUREDBORROWINGSScheduleOfSecuredBorrowingsDetails</Role>
      <ShortName>SECURED BORROWINGS - Schedule of Secured Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>100390 - Disclosure - SECURED BORROWINGS - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSECUREDBORROWINGSAdditionalInformationDetails</Role>
      <ShortName>SECURED BORROWINGS - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>100400 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENTAdditionalInformationDetails</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>100410 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Schedule of Swap Agreements Expiration Period and Characteristics (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENTScheduleOfSwapAgreementsExpirationPeriodAndCharacteristicsDetails</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Schedule of Swap Agreements Expiration Period and Characteristics (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>100420 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Components of Balance Sheet (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENTComponentsOfBalanceSheetDetails</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Components of Balance Sheet (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>100430 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Components of Income Statement and Other Comprehensive Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENTComponentsOfIncomeStatementAndOtherComprehensiveIncomeDetails</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Components of Income Statement and Other Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>100440 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Schedule of Offsetting of Derivative Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENTScheduleOfOffsettingOfDerivativeAssetsDetails</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Schedule of Offsetting of Derivative Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>100450 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Schedule of Offsetting of Financial Liabilities and Derivative Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENTScheduleOfOffsettingOfFinancialLiabilitiesAndDerivativeLiabilitiesDetails</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Schedule of Offsetting of Financial Liabilities and Derivative Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>100460 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Schedule of Offsetting of Financial Liabilities and Derivative Liabilities (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENTScheduleOfOffsettingOfFinancialLiabilitiesAndDerivativeLiabilitiesParentheticalDetails</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Schedule of Offsetting of Financial Liabilities and Derivative Liabilities (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>100470 - Disclosure - USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Changes in Accumulated Other Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUSEOFDERIVATIVESOFFSETTINGDISCLOSURESANDCHANGESINOTHERCOMPREHENSIVEINCOMEBYCOMPONENTChangesInAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Changes in Accumulated Other Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>100480 - Disclosure - UNSECURED BORROWINGS - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUNSECUREDBORROWINGSAdditionalInformationDetails</Role>
      <ShortName>UNSECURED BORROWINGS - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>100490 - Disclosure - UNSECURED BORROWINGS - Schedule of Subordinated Note Balances and Related Weighted Average Interest Rates (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureUNSECUREDBORROWINGSScheduleOfSubordinatedNoteBalancesAndRelatedWeightedAverageInterestRatesDetails</Role>
      <ShortName>UNSECURED BORROWINGS - Schedule of Subordinated Note Balances and Related Weighted Average Interest Rates (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>100500 - Disclosure - FAIR VALUES OF FINANCIAL INSTRUMENTS - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureFAIRVALUESOFFINANCIALINSTRUMENTSAdditionalInformationDetails</Role>
      <ShortName>FAIR VALUES OF FINANCIAL INSTRUMENTS - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>100510 - Disclosure - FAIR VALUES OF FINANCIAL INSTRUMENTS - Summary of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureFAIRVALUESOFFINANCIALINSTRUMENTSSummaryOfFinancialInstrumentsDetails</Role>
      <ShortName>FAIR VALUES OF FINANCIAL INSTRUMENTS - Summary of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>100520 - Disclosure - FAIR VALUES OF FINANCIAL INSTRUMENTS - Fair Value and Related Disclosures for Debt Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureFAIRVALUESOFFINANCIALINSTRUMENTSFairValueAndRelatedDisclosuresForDebtSecuritiesDetails</Role>
      <ShortName>FAIR VALUES OF FINANCIAL INSTRUMENTS - Fair Value and Related Disclosures for Debt Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>100530 - Disclosure - FAIR VALUES OF FINANCIAL INSTRUMENTS - Securities in Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureFAIRVALUESOFFINANCIALINSTRUMENTSSecuritiesInUnrealizedLossPositionDetails</Role>
      <ShortName>FAIR VALUES OF FINANCIAL INSTRUMENTS - Securities in Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>100540 - Disclosure - INCOME TAXES - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureINCOMETAXESAdditionalInformationDetails</Role>
      <ShortName>INCOME TAXES - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>100550 - Disclosure - INCOME TAXES - Schedule of Effective Income Tax Rate Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureINCOMETAXESScheduleOfEffectiveIncomeTaxRateReconciliationDetails</Role>
      <ShortName>INCOME TAXES - Schedule of Effective Income Tax Rate Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>100560 - Disclosure - INCOME TAXES - Components of Deferred Tax Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureINCOMETAXESComponentsOfDeferredTaxAssetsAndLiabilitiesDetails</Role>
      <ShortName>INCOME TAXES - Components of Deferred Tax Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>100570 - Disclosure - STOCKHOLDERS' EQUITY - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSTOCKHOLDERSEQUITYAdditionalInformationDetails</Role>
      <ShortName>STOCKHOLDERS' EQUITY - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>100580 - Disclosure - STOCKHOLDERS' EQUITY - Schedule of Shares Repurchased, Repurchase Price and Capital Deployed (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSTOCKHOLDERSEQUITYScheduleOfSharesRepurchasedRepurchasePriceAndCapitalDeployedDetails</Role>
      <ShortName>STOCKHOLDERS' EQUITY - Schedule of Shares Repurchased, Repurchase Price and Capital Deployed (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>100590 - Disclosure - EQUITY INCENTIVE PLAN - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureEQUITYINCENTIVEPLANAdditionalInformationDetails</Role>
      <ShortName>EQUITY INCENTIVE PLAN - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>100600 - Disclosure - EQUITY INCENTIVE PLAN - Schedule of Performance-Based Restricted Stock Units Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureEQUITYINCENTIVEPLANScheduleOfPerformanceBasedRestrictedStockUnitsActivityDetails</Role>
      <ShortName>EQUITY INCENTIVE PLAN - Schedule of Performance-Based Restricted Stock Units Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>100610 - Disclosure - EQUITY INCENTIVE PLAN - Schedule of Stock Award Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureEQUITYINCENTIVEPLANScheduleOfStockAwardActivityDetails</Role>
      <ShortName>EQUITY INCENTIVE PLAN - Schedule of Stock Award Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>100620 - Disclosure - QUARTERLY RESULTS (UNAUDITED) - Summarized Quarterly Results of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureQUARTERLYRESULTSUNAUDITEDSummarizedQuarterlyResultsOfOperationsDetails</Role>
      <ShortName>QUARTERLY RESULTS (UNAUDITED) - Summarized Quarterly Results of Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://capstead.com/20191231/taxonomy/role/DisclosureQUARTERLYRESULTSTables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="cmo-10k_20191231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>100630 - Disclosure - SUBSEQUENT EVENTS - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://capstead.com/20191231/taxonomy/role/DisclosureSUBSEQUENTEVENTSAdditionalInformationDetails</Role>
      <ShortName>SUBSEQUENT EVENTS - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-K" original="cmo-10k_20191231.htm">cmo-10k_20191231.htm</File>
    <File>cmo-20191231.xsd</File>
    <File>cmo-20191231_cal.xml</File>
    <File>cmo-20191231_def.xml</File>
    <File>cmo-20191231_lab.xml</File>
    <File>cmo-20191231_pre.xml</File>
    <File>cmo-ex23_6.htm</File>
    <File>cmo-ex311_10.htm</File>
    <File>cmo-ex312_8.htm</File>
    <File>cmo-ex321_7.htm</File>
    <File>cmo-ex322_9.htm</File>
    <File>cmo-ex46_156.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>grfddleuafqm000001.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
