| Cover Page | Dec. 31, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Dec. 31, 2020 |
| Entity Registrant Name | Western Asset Mortgage Capital Corporation |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-35543 |
| Entity Tax Identification Number | 27-0298092 |
| Entity Address, Address Line One | 385 East Colorado Boulevard |
| Entity Address, City or Town | Pasadena |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 91101 |
| City Area Code | 626 |
| Local Phone Number | 844-9400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common Stock, $0.01 par value |
| Trading Symbol | WMC |
| Security Exchange Name | NYSE |
| Entity Central Index Key | 0001465885 |
| Amendment Flag | false |
