Agreement period of LIBOR interest rate, in PnYnMnDTnHnMnS' format.

No definition available.

Name:	cmo_LiborInterestRateAgreementTerm
Namespace Prefix:	cmo_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Average interest rate on new forward-starting SWAP agreements entered into during period specified by the derivative(s).

+ References

No definition available.

+ Details

Name:	cmo_SwapAgreementAverageInterestRateDuringPeriod
Namespace Prefix:	cmo_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Average interest rate on SWAP agreements expiring during period specified by the derivative(s).

+ References

No definition available.

+ Details

Name:	cmo_SwapAgreementAverageInterestRateExpiringDuringPeriod
Namespace Prefix:	cmo_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate notional amount entered into during period specified by the derivative(s). Expressed as an absolute value.

+ References

No definition available.

+ Details

Name:	cmo_SwapAgreementNotionalAmountDuringPeriod
Namespace Prefix:	cmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate notional amount expiring during period specified by the derivative(s). Expressed as an absolute value.

+ References

No definition available.

+ Details

Name:	cmo_SwapAgreementNotionalAmountExpiringDuringPeriod
Namespace Prefix:	cmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The estimated value of gains (losses), net anticipated to be transferred in the future from accumulated other comprehensive income into earnings.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_NotionalAmountOfDerivativesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying value as of the balance sheet date of obligations incurred through that date and payable arising from transactions not otherwise specified in the taxonomy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_OtherAccruedLiabilitiesCurrentAndNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=cmo_CurrentlyPayingContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=cmo_SecondQuarter2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=cmo_ThirdQuarter2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=cmo_FourthQuarter2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=cmo_FirstQuarter2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=cmo_SecondQuarter2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=cmo_ThirdQuarter2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=cmo_FourthQuarter2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=cmo_FirstQuarter2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=cmo_ForwardStartingContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeByNatureAxis=cmo_SecondQuarter2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_SwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=cmo_RepurchaseArrangementsAndSimilarBorrowingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT- Schedule of Swap Agreements (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2010	Dec. 31, 2015
Notional Disclosures [Abstract]
Notional amount	$ 8,200,000
Derivative instruments unrealized losses to be recognized	13,500
Interest Rate SWAP Currently-Paying Contracts [Member]
Notional Disclosures [Abstract]
Notional amount	8,100,000
Interest Rate SWAP Currently-Paying Contracts [Member] \| Second Quarter 2016 (expired April 1, 2016)
Notional Disclosures [Abstract]
Notional amount	$ 1,100,000
Average-fixed rate payment requirement	0.47%
Interest Rate SWAP Currently-Paying Contracts [Member] \| Third Quarter 2016 [Member]
Notional Disclosures [Abstract]
Notional amount	$ 700,000
Average-fixed rate payment requirement	0.56%
Interest Rate SWAP Currently-Paying Contracts [Member] \| Fourth Quarter 2016 [Member]
Notional Disclosures [Abstract]
Notional amount	$ 800,000
Average-fixed rate payment requirement	0.66%
Interest Rate SWAP Currently-Paying Contracts [Member] \| First Quarter 2017 [Member]
Notional Disclosures [Abstract]
Notional amount	$ 1,000,000
Average-fixed rate payment requirement	0.72%
Interest Rate SWAP Currently-Paying Contracts [Member] \| Second Quarter 2017 [Member]
Notional Disclosures [Abstract]
Notional amount	$ 900,000
Average-fixed rate payment requirement	0.74%
Interest Rate SWAP Currently-Paying Contracts [Member] \| Third Quarter 2017 [Member]
Notional Disclosures [Abstract]
Notional amount	$ 400,000
Average-fixed rate payment requirement	0.74%
Interest Rate SWAP Currently-Paying Contracts [Member] \| Fourth Quarter 2017 [Member]
Notional Disclosures [Abstract]
Notional amount	$ 1,500,000
Average-fixed rate payment requirement	0.79%
Interest Rate SWAP Currently-Paying Contracts [Member] \| First Quarter 2018 [Member]
Notional Disclosures [Abstract]
Notional amount	$ 1,700,000
Average-fixed rate payment requirement	0.76%
Interest Rate SWAP Forward-Starting Contracts [Member]
Derivative Instruments And Hedging Activities [Line Items]
Swap agreement notional amount expiring during period	$ 300,000
SWAP agreement average interest rate expiring during period	0.92%
Interest Rate SWAP Forward-Starting Contracts [Member] \| Second Quarter 2018 [Member]
Notional Disclosures [Abstract]
Notional amount	$ 100,000
Average-fixed rate payment requirement	0.77%
Interest Rate Swap Agreements [Member]
Derivative Instruments And Hedging Activities [Line Items]
SWAP agreement notional amount during period	$ 1,500,000	$ 100,000
SWAP agreement average interest rate during period	0.73%	4.09%
Swap agreement notional amount expiring during period	$ 1,700,000
SWAP agreement average interest rate expiring during period	0.51%
Notional Disclosures [Abstract]
Payment term of LIBOR interest rate agreement	2 years	20 years
Accrued interest	$ 10,700		$ 10,900
Portfolio-Related Secured Borrowings [Member]
Notional Disclosures [Abstract]
Accrued interest	$ 8,200		$ 9,300