SECURED BORROWINGS - Additional Information (Details) - USD ($) $ in Billions	3 Months Ended	12 Months Ended
Jun. 30, 2021	Dec. 31, 2020
Disclosure Of Repurchase Agreements [Abstract]
Average secured borrowings	$ 6.6	$ 7.4

Average borrowing amount outstanding during the period.

No definition available.

Name:	cmo_AverageBorrowingsAdjustedAmount
Namespace Prefix:	cmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration