Document And Entity Information	9 Months Ended
	Sep. 30, 2012	Oct. 31, 2012
Document and Entity Information [Abstract]
Entity Registrant Name	MISSION WEST PROPERTIES INC
Document Type	10-Q
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		22,674,020
Amendment Flag	false
Entity Central Index Key	0001067419
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Well-known Seasoned Issuer	No
Document Period End Date	Sep. 30, 2012
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q3