Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Nov. 09, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3
Entity Registrant Name	Mast Therapeutics, Inc.
Entity Central Index Key	0001160308
Trading Symbol	MSTX
Current Fiscal Year End Date	--12-31
Entity Filer Category	Smaller Reporting Company
Entity Common Stock, Shares Outstanding		163,614,297