Document and Entity Information	0 Months Ended
	Dec. 31, 2012
Risk/Return:
Document Type	485BPOS
Document Period End Date	Dec. 31, 2012
Registrant Name	MFS VARIABLE INSURANCE TRUST III
Central Index Key	0001065698
Amendment Flag	false
Document Creation Date	Apr. 29, 2013
Document Effective Date	Apr. 30, 2013
Prospectus Date	Apr. 30, 2013