SECURED BORROWINGS - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Disclosure Of Repurchase Agreements [Abstract]
Average secured borrowings	$ 7,450	$ 10,200

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