Document And Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Jul. 26, 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 Common Stock, Shares Outstanding (in shares)		42,299,700
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Amendment Flag	false