| Document And Entity Information | Oct. 22, 2019 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 22, 2019 |
| Entity Registrant Name | PACCAR INC |
| Entity Central Index Key | 0000075362 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-14817 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 91-0351110 |
| Entity Address, Address Line One | 777 106th Avenue NE |
| Entity Address, City or Town | Bellevue |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 98004 |
| City Area Code | 425 |
| Local Phone Number | 468-7400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of Each Class | Common stock, $1 par value |
| Trading Symbol(s) | PCAR |
| Name of Each Exchange on Which Registered | NASDAQ |
