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 number of months until coupon reset. Current reset ARM securities have a months to roll of less than 18 months, while longer-to-reset ARM securities have a months to roll of 18 months or greater.

No definition available.

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

The average number of months until coupon reset. Current reset ARM securities have a months to roll of less that 18 months, while longer-to-reset ARM securities have a months to roll of 18 months or greater.

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.

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

Name:	invest_InvestmentAxis=cmo_CurrentResetAdjustableRateMortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	invest_InvestmentAxis=cmo_LongerToResetAdjustableRateMortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: