Investments In Unconsolidated Affiliates (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012
Investments in and Advances to Affiliates [Line Items]
Issuance of common securities	$ 3.1

Cash outflow for common stock issued to the Company from the statutory trusts not consolidated under VIE rules, therefore accounted for under the equity method.

No definition available.

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