Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Nov. 03, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	CAPSTEAD MORTGAGE CORP
Entity Central Index Key	0000766701
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		96,104,662
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
Trading Symbol	CMO
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017