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COMPENSATION PROGRAMS - Schedule of Restricted Stock Awards (Details) (USD $)
12 Months Ended 1 Months Ended 12 Months Ended 0 Months Ended 12 Months Ended 1 Months Ended
Dec. 31, 2014
Jan. 31, 2014
Dec. 31, 2013
Jul. 31, 2014
Apr. 30, 2014
Dec. 31, 2012
Jan. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total original grants (in shares) 72,237us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod            
Grant date fair value per share (in dollars per share) $ 12.80us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue            
Equity instruments other than options, nonvested, number of shares [Roll Forward]              
Unvested stock awards outstanding at beginning of period (in shares) 528,931us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber 528,931us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber          
Grants (in shares) 72,237us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod            
Vestings (in shares) (164,587)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod            
Unvested stock awards outstanding at end of period (in shares) 436,581us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber            
Equity instruments other than options, nonvested, weighted average grant date fair value [Abstract]              
Unvested stock awards outstanding at beginning of year (in dollars per share) $ 12.51us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue 12.51us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue          
Grants (in dollars per share) $ 12.80us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue            
Vestings (in dollars per share) $ 13.23us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue            
Unvested stock awards outstanding at end of year (in dollars per share) $ 12.29us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue            
Common stock dividend payable $ 34,054,000us-gaap_DividendsPayableCurrentAndNoncurrent   $ 30,872,000us-gaap_DividendsPayableCurrentAndNoncurrent        
Service-Based Stock Awards [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total original grants (in shares) 37,237us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
22,164us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
35,703us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
       
Grant date fair value per share (in dollars per share) $ 12.47us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
  $ 12.34us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
       
Equity instruments other than options, nonvested, number of shares [Roll Forward]              
Grants (in shares) 37,237us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
22,164us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
35,703us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
       
Equity instruments other than options, nonvested, weighted average grant date fair value [Abstract]              
Grants (in dollars per share) $ 12.47us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
  $ 12.34us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
       
Service-Based Stock Awards [Member] | Director Stock Award [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total original grants (in shares)       35,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
/ us-gaap_TitleOfIndividualAxis
= cmo_DirectorStockAwardMember
28,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
/ us-gaap_TitleOfIndividualAxis
= cmo_DirectorStockAwardMember
   
Grant date fair value per share (in dollars per share)       $ 13.16us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
/ us-gaap_TitleOfIndividualAxis
= cmo_DirectorStockAwardMember
$ 13.02us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
/ us-gaap_TitleOfIndividualAxis
= cmo_DirectorStockAwardMember
   
Equity instruments other than options, nonvested, number of shares [Roll Forward]              
Grants (in shares)       35,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
/ us-gaap_TitleOfIndividualAxis
= cmo_DirectorStockAwardMember
28,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
/ us-gaap_TitleOfIndividualAxis
= cmo_DirectorStockAwardMember
   
Equity instruments other than options, nonvested, weighted average grant date fair value [Abstract]              
Grants (in dollars per share)       $ 13.16us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
/ us-gaap_TitleOfIndividualAxis
= cmo_DirectorStockAwardMember
$ 13.02us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_ServiceSharesMember
/ us-gaap_TitleOfIndividualAxis
= cmo_DirectorStockAwardMember
   
Stock Awards Activity [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total original grants (in shares) 114,423us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
  121,026us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
    67,599us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
 
Grant date fair value per share (in dollars per share) $ 13.31us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
  $ 12.01us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
    $ 10.18us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
 
Equity instruments other than options, nonvested, number of shares [Roll Forward]              
Grants (in shares) 114,423us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
  121,026us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
    67,599us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
 
Equity instruments other than options, nonvested, weighted average grant date fair value [Abstract]              
Grants (in dollars per share) $ 13.31us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
  $ 12.01us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
    $ 10.18us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
 
Long term incentive compensation expense 1,500,000cmo_LongTermIncentiveCompensationExpense
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
  1,800,000cmo_LongTermIncentiveCompensationExpense
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
    1,900,000cmo_LongTermIncentiveCompensationExpense
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
 
Other general and administrative expense 315,000us-gaap_OtherSellingGeneralAndAdministrativeExpense
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
  387,000us-gaap_OtherSellingGeneralAndAdministrativeExpense
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
    362,000us-gaap_OtherSellingGeneralAndAdministrativeExpense
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
 
Common stock dividend payable 1,400,000us-gaap_DividendsPayableCurrentAndNoncurrent
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
  1,200,000us-gaap_DividendsPayableCurrentAndNoncurrent
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
       
Total of unrecognized compensation expense for unvested stock award 2,200,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= cmo_StockAwardsActivityMember
           
Compensation cost not yet recognized, period for recognition 1 year 6 months            
Long Term Equity Based Awards [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total original grants (in shares)     242,505us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
       
Grant date fair value per share (in dollars per share) $ 7.21us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
  $ 12.45us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
       
Equity instruments other than options, nonvested, number of shares [Roll Forward]              
Grants (in shares)     242,505us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
       
Equity instruments other than options, nonvested, weighted average grant date fair value [Abstract]              
Grants (in dollars per share) $ 7.21us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
  $ 12.45us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
       
Long term incentive compensation expense     582,000cmo_LongTermIncentiveCompensationExpense
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
       
Common stock dividend payable 213,000us-gaap_DividendsPayableCurrentAndNoncurrent
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
  75,000us-gaap_DividendsPayableCurrentAndNoncurrent
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
       
Total estimated compensation expense for restricted stock units 1,700,000cmo_TotalEstimatedCompensationExpenseForRestrictedStockUnits
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
  3,000,000cmo_TotalEstimatedCompensationExpenseForRestrictedStockUnits
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
       
Long Term Equity Based Awards [Member] | Subsequent Event [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total original grants (in shares)             247,512us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Grant date fair value per share (in dollars per share)             $ 8.83us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Equity instruments other than options, nonvested, number of shares [Roll Forward]              
Grants (in shares)             247,512us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Equity instruments other than options, nonvested, weighted average grant date fair value [Abstract]              
Grants (in dollars per share)             $ 8.83us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Total of unrecognized compensation expense for unvested stock award             $ 2,200,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Long Term Equity Based Awards [Member] | February 2015 [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total original grants (in shares) 125,221us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= cmo_February2015Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Grant date fair value per share (in dollars per share) $ 12.58us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardDateAxis
= cmo_February2015Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Equity instruments other than options, nonvested, number of shares [Roll Forward]              
Grants (in shares) 125,221us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= cmo_February2015Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Equity instruments other than options, nonvested, weighted average grant date fair value [Abstract]              
Grants (in dollars per share) $ 12.58us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardDateAxis
= cmo_February2015Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Long Term Equity Based Awards [Member] | January 2016 [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total original grants (in shares) 133,571us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= cmo_January2016Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Grant date fair value per share (in dollars per share) $ 12.17us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardDateAxis
= cmo_January2016Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Equity instruments other than options, nonvested, number of shares [Roll Forward]              
Grants (in shares) 133,571us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= cmo_January2016Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Equity instruments other than options, nonvested, weighted average grant date fair value [Abstract]              
Grants (in dollars per share) $ 12.17us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardDateAxis
= cmo_January2016Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Long Term Equity Based Awards [Member] | January 2017 [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total original grants (in shares) 69,849us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= cmo_January2017Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Grant date fair value per share (in dollars per share) $ 11.67us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardDateAxis
= cmo_January2017Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Equity instruments other than options, nonvested, number of shares [Roll Forward]              
Grants (in shares) 69,849us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardDateAxis
= cmo_January2017Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember
           
Equity instruments other than options, nonvested, weighted average grant date fair value [Abstract]              
Grants (in dollars per share) $ 11.67us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardDateAxis
= cmo_January2017Member
/ us-gaap_AwardTypeAxis
= cmo_LongTermPerformanceSharesMember