Document And Entity Information (USD $)	12 Months Ended
	Dec. 31, 2011	Feb. 29, 2012	Jun. 30, 2011
Entity Information
Entity Registrant Name	MISSION WEST PROPERTIES INC
Entity Central Index Key	0001067419
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Common Stock, Shares Outstanding		22,668,020
Entity Public Float			$ 177,454,951
Document Information
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2011
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY