v3.26.1
Label Element Value
Risk/Return: oef_RiskReturnAbstract  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Apr. 30, 2026
Registrant Name dei_EntityRegistrantName MFS® SERIES TRUST XII
Entity Central Index Key dei_EntityCentralIndexKey 0001330967
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate Aug. 27, 2026
Document Effective Date dei_DocumentEffectiveDate Aug. 28, 2026
Entity Investment Company Type dei_EntityInvCompanyType N-1A
Prospectus Date oef_ProspectusDate Aug. 28, 2026
MFS Core Bond Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MCBEX
MFS Core Bond Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MCBCX
MFS Core Bond Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MCBFX
MFS Core Bond Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MCBGX
MFS Core Bond Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MCBHX
MFS Core Bond Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MCBJX
MFS Core Bond Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MCBKX
MFS Core Bond Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MCBMX
MFS Lifetime Income Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLLAX
MFS Lifetime Income Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLLBX
MFS Lifetime Income Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLLCX
MFS Lifetime Income Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLLIX
MFS Lifetime Income Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLLEX
MFS Lifetime Income Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLLGX
MFS Lifetime Income Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLLHX
MFS Lifetime Income Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLLJX
MFS Lifetime Income Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLLKX
MFS Lifetime 2030 Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLTAX
MFS Lifetime 2030 Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLTBX
MFS Lifetime 2030 Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLTCX
MFS Lifetime 2030 Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLTIX
MFS Lifetime 2030 Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLTEX
MFS Lifetime 2030 Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLTGX
MFS Lifetime 2030 Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLTHX
MFS Lifetime 2030 Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLTJX
MFS Lifetime 2030 Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLTKX
MFS Lifetime 2035 Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFEAX
MFS Lifetime 2035 Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFEBX
MFS Lifetime 2035 Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFECX
MFS Lifetime 2035 Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFEDX
MFS Lifetime 2035 Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFERX
MFS Lifetime 2035 Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFESX
MFS Lifetime 2035 Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFETX
MFS Lifetime 2035 Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFEUX
MFS Lifetime 2035 Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFEKX
MFS Lifetime 2040 Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLFAX
MFS Lifetime 2040 Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLFBX
MFS Lifetime 2040 Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLFCX
MFS Lifetime 2040 Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLFIX
MFS Lifetime 2040 Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLFEX
MFS Lifetime 2040 Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLFGX
MFS Lifetime 2040 Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLFHX
MFS Lifetime 2040 Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLFJX
MFS Lifetime 2040 Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MLFKX
MFS Lifetime 2045 Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LTMAX
MFS Lifetime 2045 Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LTMBX
MFS Lifetime 2045 Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LTMDX
MFS Lifetime 2045 Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LTMKX
MFS Lifetime 2045 Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LTMRX
MFS Lifetime 2045 Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LTMSX
MFS Lifetime 2045 Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LTMTX
MFS Lifetime 2045 Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LTMUX
MFS Lifetime 2045 Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LTMLX
MFS Lifetime 2050 Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFFSX
MFS Lifetime 2050 Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFFRX
MFS Lifetime 2050 Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFFDX
MFS Lifetime 2050 Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFFIX
MFS Lifetime 2050 Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFFMX
MFS Lifetime 2050 Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFFNX
MFS Lifetime 2050 Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFFOX
MFS Lifetime 2050 Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFFPX
MFS Lifetime 2050 Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFFKX
MFS Lifetime 2055 Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFIAX
MFS Lifetime 2055 Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFIBX
MFS Lifetime 2055 Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFICX
MFS Lifetime 2055 Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFIIX
MFS Lifetime 2055 Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFIRX
MFS Lifetime 2055 Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFISX
MFS Lifetime 2055 Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFITX
MFS Lifetime 2055 Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFIUX
MFS Lifetime 2055 Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFIKX
MFS Lifetime 2060 Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFJAX
MFS Lifetime 2060 Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFJBX
MFS Lifetime 2060 Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFJCX
MFS Lifetime 2060 Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFJIX
MFS Lifetime 2060 Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFJEX
MFS Lifetime 2060 Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFJGX
MFS Lifetime 2060 Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFJTX
MFS Lifetime 2060 Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFJUX
MFS Lifetime 2060 Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MFJKX
MFS Lifetime 2065 Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFTFX
MFS Lifetime 2065 Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFTGX
MFS Lifetime 2065 Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFTHX
MFS Lifetime 2065 Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFTJX
MFS Lifetime 2065 Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFTKX
MFS Lifetime 2065 Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFTLX
MFS Lifetime 2065 Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFTMX
MFS Lifetime 2065 Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol LFTNX