| Document and Entity Information | Aug. 05, 2021 |
| Document And Entity Information [Line Items] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001464423 |
| Document Type | 8-K |
| Document Period End Date | Aug. 5, 2021 |
| Entity Registrant Name | PennyMac Mortgage Investment Trust |
| Entity Incorporation State Country Code | MD |
| Entity File Number | 001-34416 |
| Entity Tax Identification Number | 27-0186273 |
| Entity Address, Address Line One | 3043 Townsgate Road |
| Entity Address, City or Town | Westlake Village |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 91361 |
| City Area Code | (818) |
| Local Phone Number | 224-7442 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Shares of Beneficial Interest, $0.01 par value |
| Trading Symbol | PMT |
| Security Exchange Name | NYSE |
| Series A Preferred Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 8.125% Series A Cumulative Redeemable Preferred Shares of Beneficial Interest, $0.01 par value |
| Trading Symbol | PMT/PA |
| Security Exchange Name | NYSE |
| Series B Preferred Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 8.00% Series B Cumulative Redeemable Preferred Shares of Beneficial Interest, $0.01 par value |
| Trading Symbol | PMT/PB |
| Security Exchange Name | NYSE |
