SECURED BORROWINGS - Schedule of Average Borrowings Outstanding (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Disclosure Of Repurchase Agreements [Abstract]
Average borrowings for the effects of related derivatives	$ 11,156,608	$ 11,439,646
Effective borrowing rate	2.12%	2.07%

Average borrowings rates adjusted for effects related derivatives.

No definition available.

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Namespace Prefix:	cmo_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant