| Document And Entity Information | Sep. 14, 2020 |
| Document Information [Line Items] | |
| Entity, Registrant Name | MONOLITHIC POWER SYSTEMS, INC. |
| Document, Type | 8-K |
| Document, Period End Date | Sep. 14, 2020 |
| Entity, Incorporation, State or Country Code | DE |
| Entity, File Number | 000-51026 |
| Entity, Tax Identification Number | 77-0466789 |
| Entity, Address, Address Line One | 5808 Lake Washington Blvd. NE |
| Entity, Address, City or Town | Kirkland |
| Entity, Address, State or Province | WA |
| Entity, Address, Postal Zip Code | 98033 |
| City Area Code | 425 |
| Local Phone Number | 296-9956 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | MPWR |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001280452 |
