ACCOUNTING POLICIES - Additional Information (Details)	12 Months Ended
Dec. 31, 2020
Accounting Policies [Abstract]
Percentage of taxable income to be distributed	100.00%

Represents the percentage of taxable income to be distributed.

No definition available.

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