| Document and Entity Information | May 04, 2021 |
| Document and Entity Information [Line Items] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | May 4, 2021 |
| Entity Registrant Name | CAPSTEAD MORTGAGE CORPORATION |
| Entity Central Index Key | 0000766701 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-08896 |
| Entity Incorporation, State or Country Code | MD |
| 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 |
| City Area Code | 214 |
| Local Phone Number | 874-2323 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Common Stock ($0.01 par value) [Member] | |
| Document and Entity Information [Line Items] | |
| Title of each class | Common Stock ($0.01 par value) |
| Trading Symbols | CMO |
| Name of each exchange on which registered | NYSE |
| Series E Preferred Stock [Member] | |
| Document and Entity Information [Line Items] | |
| Title of each class | 7.50% Series E Cumulative Redeemable Preferred Stock ($0.10 par value) |
| Trading Symbols | CMOPRE |
| Name of each exchange on which registered | NYSE |
