Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Jul. 26, 2019
Entity Registrant Name	CAPSTEAD MORTGAGE CORP
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
Entity Common Stock, Shares Outstanding		85,606,455
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	CMO
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Entity File Number	001-08896
Entity Tax Identification Number	752027937
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
Cumulative Redeemable Preferred Stock, Series E [Member]
Trading Symbol	CMOPRE