SECURED BORROWINGS - Schedule of Average Borrowings Outstanding (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Repurchase Agreements [Abstract]
Average borrowings for the effects of related derivatives held as cash flow hedges	$ 11,884,834	$ 12,388,557
Effective borrowing rate considering cash flow hedges	1.74%	1.12%

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

No definition available.

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