QUARTERLY RESULTS (UNAUDITED) - Summarized Quarterly Results of Operations (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016
Quarterly Financial Information Disclosure [Abstract]
Separation of service charge		$ 3.0	$ 3.0
Alternative minimum tax credit refunds	$ 1.9

Estimated costs associated with employees' death, retirement, or otherwise termination of employment.

No definition available.

Name:	cmo_SeparationOfServiceCharge
Namespace Prefix:	cmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration