Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 19, 2021	Jun. 30, 2020
Cover [Abstract]
Entity Registrant Name	CAPSTEAD MORTGAGE CORPORATION
Entity Central Index Key	0000766701
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Document Transition Report	false
Document Annual Report	true
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 518,208,138
Entity Common Stock, Shares Outstanding		96,782,617
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
ICFR Auditor Attestation Flag	true
Entity Tax Identification Number	75-2027937
Entity File Number	001-08896
Entity Address, Address Line One	8401 North Central Expressway
Entity Address, Address Line Two	Suite 800
Entity Address, City or Town	Dallas
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	75225-4404
Entity Incorporation, State or Country Code	MD
Entity Interactive Data Current	Yes
City Area Code	214
Local Phone Number	874-2323
Documents Incorporated by Reference [Text Block]	Portions of the Registrant’s definitive Proxy Statement, to be issued in connection with the 2021 Annual Meeting of Stockholders of the Registrant, are incorporated by reference into Part III.
Common Stock [Member]
Cover [Abstract]
Trading Symbol	CMO
Security Exchange Name	NYSE
Title of 12(b) Security	Common Stock ($0.01 par value)
Cumulative Redeemable Preferred Stock, Series E [Member]
Cover [Abstract]
Trading Symbol	CMOPRE
Security Exchange Name	NYSE
Title of 12(b) Security	7.50% Series E Cumulative Redeemable        Preferred Stock ($0.10 par value)