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

This item represents the cost of longer-to-reset debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

No definition available.

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

The average period until coupon resets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

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

The period until coupon resets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

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

Represents the weighted average contractual maturity period of the portfolio without giving affect to mortgage prepayments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

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

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

Name:	us-gaap_FinancialInstrumentAxis=cmo_CurrentResetAdjustableRateMortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancialInstrumentAxis=cmo_LongerToResetAdjustableRateMortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: