Name:	cmo_AccruedInterestReceivable
Namespace Prefix:	cmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Average interest rate on the amount to be repaid to the counterparty to the secured borrowing.

No definition available.

Name:	cmo_AverageInterestRate
Namespace Prefix:	cmo_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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.

No definition available.

Name:	cmo_SecuredBorrowingsIncludingInterestRateHedgingActivityLineItems
Namespace Prefix:	cmo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for repurchase agreements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

Name:	us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForRepurchaseAgreements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount, before effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=cmo_QuarterEndBorrowingRatesAdjustedForEffectsOfRelatedDerivativesHeldAsCashFlowHedgesMember
Namespace Prefix:
Data Type:	na
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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityUpTo30DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=cmo_BorrowingsWithMaturitiesOf31To90DaysMember
Namespace Prefix:
Data Type:	na
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