Annual incentive compensation payable at the end of the period.

No definition available.

Name:	cmo_AnnualIncentiveCompensationProgramAccruals
Namespace Prefix:	cmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Grant date.

+ References

No definition available.

+ Details

Name:	cmo_GrantDate
Namespace Prefix:	cmo_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Long-term incentive compensation expense during the period relating to amortization of the grant date fair value of employee performance-based and service-based stock awards.

+ References

No definition available.

+ Details

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

- Definition

Long-term incentive compensation expense during the period relating to amortization of the grant date fair value of employee performance-based restricted stock units.

+ References

No definition available.

+ Details

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

- Definition

Share based compensation arrangement by share based payment award award vesting period.

+ References

No definition available.

+ Details

Name:	cmo_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod
Namespace Prefix:	cmo_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

The period of time over which the employee needs to meet the performance objectives in order to qualify for the stock unit award in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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

- Definition

Refers to an officer that has a targeted award opportunity percentage on their base salary.

+ References

No definition available.

+ Details

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

- Definition

Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(20))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(15)(5))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

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

- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense for salary, wage, profit sharing; incentive and equity-based compensation; and other employee benefit.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(b)(4))
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

+ Details

Name:	us-gaap_LaborAndRelatedExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of selling, general and administrative expense classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.4)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

+ Details

Name:	us-gaap_OtherSellingGeneralAndAdministrativeExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

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

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

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

- Details

Name:	us-gaap_AwardTypeAxis=cmo_LongTermEquityBasedAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=cmo_AnnualIncentiveCompensationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=cmo_DividendEquivalentRightsProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_AccountsPayableAndAccruedLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=cmo_PerformanceBasedRestrictedStockUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=cmo_ServiceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_ManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=cmo_EmployeeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=cmo_StockAwardsActivityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

COMPENSATION PROGRAMS - Additional Information (Details) - USD ($)	1 Months Ended						3 Months Ended		9 Months Ended
	Jul. 31, 2018	Jan. 31, 2018	Jul. 31, 2017	Jan. 31, 2017	Feb. 29, 2016	Jan. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Short-term incentive compensation program accruals							$ 1,100,000		$ 1,100,000
Annual incentive compensation expense							1,913,000	$ 1,073,000	5,521,000	$ 4,021,000
Dividend equivalent rights payable							$ 10,365,000		$ 10,365,000		$ 18,487,000
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Targeted award opportunity on base salary									75.00%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Targeted award opportunity on base salary									125.00%
Long-Term Equity-Based Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares available for future issuances (in shares)							3,312,051		3,312,051
Annual Incentive Compensation [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual incentive compensation expense							$ 332,000		$ 1,100,000
Dividend Equivalent Rights Program [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual incentive compensation expense							66,000		246,000
Dividend Equivalent Rights Program [Member] \| Accounts Payable and Accrued Liabilities [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend equivalent rights payable							66,000		$ 66,000
Performance-based RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share awards performance period									3 years
Total original grants (in shares)		183,137		148,894	269,354
Grant date fair value per share (in dollars per share)		$ 8.71		$ 10.52	$ 8.03
Long term incentive compensation expense							211,000		$ 228,000
Service-Based Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total original grants (in shares)	57,113		41,797
Grant date fair value per share (in dollars per share)	$ 8.58		$ 10.05
Grant date	Jul. 31, 2019		Jul. 31, 2017
Service-based stock awards issuance period									1 year
Service-Based Stock Awards [Member] \| Management [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total original grants (in shares)		126,451		74,446	67,337
Grant date fair value per share (in dollars per share)		$ 8.60		$ 10.41	$ 9.32
Share-based compensation arrangement by share-based payment award vesting period		2021-01		2020-01	2019-02
Service-Based Stock Awards [Member] \| Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total original grants (in shares)		57,283		49,416		61,272
Grant date fair value per share (in dollars per share)		$ 8.60		$ 10.41		$ 7.87
Share-based compensation arrangement by share-based payment award vesting period		2021-01		2020-01		2019-01
Stock Awards Activity [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total original grants (in shares)									240,847
Grant date fair value per share (in dollars per share)									$ 8.60
Long term incentive compensation expense							307,000		$ 922,000
Other general and administrative expense							117,000		327,000
Total of unrecognized compensation expense for unvested stock award							$ 2,200,000		$ 2,200,000
Compensation cost not yet recognized, period for recognition									1 year 3 months 18 days