v3.26.1
Form N-1A Cover
Apr. 30, 2026
Prospectus [Line Items]  
Document Type 485BPOS
Amendment Flag false
Registrant Name MFS® SERIES TRUST XII
Entity Central Index Key 0001330967
Entity Investment Company Type N-1A
Document Period End Date Apr. 30, 2026
Prospectus Date May 27, 2026