Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2020	Aug. 04, 2020
Cover [Abstract]
Entity Registrant Name	CAPSTEAD MORTGAGE CORPORATION
Entity Central Index Key	0000766701
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Document Quarterly Report	true
Document Transition Report	false
Entity Common Stock, Shares Outstanding		96,468,729
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2020
Entity File Number	001-08896
Entity Tax Identification Number	75-2027937
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
City Area Code	214
Local Phone Number	874-2323
Entity Interactive Data Current	Yes
Entity Incorporation, State or Country Code	MD
Common Stock [Member]
Cover [Abstract]
Trading Symbol	CMO
Title of 12(b) Security	Common Stock ($0.01 par value)
Security Exchange Name	NYSE
Cumulative Redeemable Preferred Stock, Series E [Member]
Cover [Abstract]
Trading Symbol	CMOPRE
Title of 12(b) Security	7.50% Series E Cumulative Redeemable        Preferred Stock ($0.10 par value)
Security Exchange Name	NYSE