SECURED BORROWINGS - Schedule of Average Borrowings Outstanding (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Repurchase Agreements [Abstract]
Average borrowings for the effects of related derivatives held as cash flow hedges	$ 12,388,557	$ 12,754,022
Effective borrowing rate considering cash flow hedges	1.12%	0.84%

Average interest rate during the period adjusted for effects of related derivatives held as cash flow hedges.

No definition available.

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