Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 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 Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding (in shares)		42,412,000
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Amendment Flag	false