SECURED BORROWINGS - Schedule of Average Borrowings Outstanding (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Repurchase Agreements [Abstract]
Average borrowings for the effects of related derivatives held as cash flow hedges	$ 12,754,022	$ 13,047,509
Effective borrowing rate considering cash flow hedges	0.84%	0.66%

Average interest rate adjusted for effects of related derivatives held as cash flow hedges on the amount to be repaid to the counterparty to the repurchase agreement when the amount borrowed is different from indicated quarterly amount due to differences in timing and amount of portfolio acquisition relative to portfolio runoff.

No definition available.

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