Label	Element	Value
Risk/Return:	oef_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 31, 2025
Registrant Name	dei_EntityRegistrantName	MFS® VARIABLE INSURANCE TRUST III
Entity Central Index Key	dei_EntityCentralIndexKey	0001065698
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Apr. 29, 2026
Document Effective Date	dei_DocumentEffectiveDate	Apr. 30, 2026
Entity Investment Company Type	dei_EntityInvCompanyType	N-1A
Prospectus Date	oef_ProspectusDate	Apr. 30, 2026
MFS Blended Research Small Cap Equity Portfolio \| Initial Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Blended Research Small Cap Equity Portfolio \| Service Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Conservative Allocation Portfolio \| Initial Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Conservative Allocation Portfolio \| Service Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Global Real Estate Portfolio \| Initial Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Global Real Estate Portfolio \| Service Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Growth Allocation Portfolio \| Initial Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Growth Allocation Portfolio \| Service Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Inflation-Adjusted Bond Portfolio \| Initial Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Inflation-Adjusted Bond Portfolio \| Service Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Limited Maturity Portfolio \| Initial Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Limited Maturity Portfolio \| Service Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Mid Cap Value Portfolio \| Initial Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Mid Cap Value Portfolio \| Service Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Moderate Allocation Portfolio \| Initial Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS Moderate Allocation Portfolio \| Service Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS New Discovery Value Portfolio \| Initial Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A
MFS New Discovery Value Portfolio \| Service Class Shares
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	N/A

Boolean flag that is true when the XBRL content amends previously-filed or accepted submission.

No definition available.

Name:	dei_AmendmentFlag
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

The date the document was made available and submitted, in YYYY-MM-DD format. The date of submission, date of acceptance by the recipient, and the document effective date are all potentially different.

+ References

No definition available.

+ Details

Name:	dei_DocumentCreationDate
Namespace Prefix:	dei_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The date when a document, upon receipt and acceptance, becomes officially effective, in YYYY-MM-DD format. Usually it is a system-assigned date time value, but it may be declared by the submitter in some cases.

+ References

No definition available.

+ Details

Name:	dei_DocumentEffectiveDate
Namespace Prefix:	dei_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

For the EDGAR submission types of Form 8-K: the date of the report, the date of the earliest event reported; for the EDGAR submission types of Form N-1A: the filing date; for all other submission types: the end of the reporting or transition period. The format of the date is YYYY-MM-DD.

+ References

No definition available.

+ Details

Name:	dei_DocumentPeriodEndDate
Namespace Prefix:	dei_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The type of document being provided (such as 10-K, 10-Q, 485BPOS, etc). The document type is limited to the same value as the supporting SEC submission type, or the word 'Other'.

+ References

No definition available.

+ Details

Name:	dei_DocumentType
Namespace Prefix:	dei_
Data Type:	dei:submissionTypeItemType
Balance Type:	na
Period Type:	duration

- Definition

A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

+ Details

Name:	dei_EntityCentralIndexKey
Namespace Prefix:	dei_
Data Type:	dei:centralIndexKeyItemType
Balance Type:	na
Period Type:	duration

- Definition

One of: N-1A (Mutual Fund), N-1 (Open-End Separate Account with No Variable Annuities), N-2 (Closed-End Investment Company), N-3 (Separate Account Registered as Open-End Management Investment Company), N-4 (Variable Annuity UIT Separate Account), N-5 (Small Business Investment Company), N-6 (Variable Life UIT Separate Account), S-1 or S-3 (Face Amount Certificate Company), S-6 (UIT, Non-Insurance Product).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-T
-Number 232
-Section 313

+ Details

Name:	dei_EntityInvCompanyType
Namespace Prefix:	dei_
Data Type:	dei:invCompanyType
Balance Type:	na
Period Type:	duration

- Definition

The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

+ Details

Name:	dei_EntityRegistrantName
Namespace Prefix:	dei_
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Trading symbol of an instrument as listed on an exchange.

+ References

No definition available.

+ Details

Name:	dei_TradingSymbol
Namespace Prefix:	dei_
Data Type:	dei:tradingSymbolItemType
Balance Type:	na
Period Type:	duration

- References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form N-1A
-Section 1
-Subsection a
-Paragraph 3

+ Details

Name:	oef_ProspectusDate
Namespace Prefix:	oef_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form N-1A
-Section 1

+ Details

Name:	oef_RiskReturnAbstract
Namespace Prefix:	oef_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration