v3.10.0.1
Document And Entity Information - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2018
Feb. 20, 2019
Jun. 29, 2018
Document Information [Line Items]      
Entity Registrant Name MONOLITHIC POWER SYSTEMS INC    
Entity Central Index Key 0001280452    
Trading Symbol mpwr    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Well-known Seasoned Issuer Yes    
Entity Emerging Growth Company false    
Entity Small Business false    
Entity Common Stock, Shares Outstanding (in shares)   42,916,000  
Entity Public Float     $ 4.5
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Shell Company false