Label	Element	Value
Risk/Return:	oef_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Aug. 31, 2025
Registrant Name	dei_EntityRegistrantName	MFS® SERIES TRUST I
Entity Central Index Key	dei_EntityCentralIndexKey	0000798244
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Dec. 29, 2025
Document Effective Date	dei_DocumentEffectiveDate	Dec. 29, 2025
Entity Investment Company Type	dei_EntityInvCompanyType	N-1A
Prospectus Date	oef_ProspectusDate	Dec. 29, 2025
MFS Core Equity Fund \| Class A
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRGAX
MFS Core Equity Fund \| Class B
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRGBX
MFS Core Equity Fund \| Class C
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRGCX
MFS Core Equity Fund \| Class I
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRGRX
MFS Core Equity Fund \| Class R1
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRGGX
MFS Core Equity Fund \| Class R2
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRERX
MFS Core Equity Fund \| Class R3
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRGHX
MFS Core Equity Fund \| Class R4
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRGJX
MFS Core Equity Fund \| Class R6
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRGKX
MFS Low Volatility Equity Fund \| Class A
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MLVAX
MFS Low Volatility Equity Fund \| Class B
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MLVBX
MFS Low Volatility Equity Fund \| Class C
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MLVGX
MFS Low Volatility Equity Fund \| Class I
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MLVHX
MFS Low Volatility Equity Fund \| Class R1
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MLVMX
MFS Low Volatility Equity Fund \| Class R2
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MLVOX
MFS Low Volatility Equity Fund \| Class R3
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MLVPX
MFS Low Volatility Equity Fund \| Class R4
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MLVRX
MFS Low Volatility Equity Fund \| Class R6
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MLVTX
MFS Low Volatility Global Equity Fund \| Class A
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVGAX
MFS Low Volatility Global Equity Fund \| Class B
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVGBX
MFS Low Volatility Global Equity Fund \| Class C
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVGCX
MFS Low Volatility Global Equity Fund \| Class I
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVGIX
MFS Low Volatility Global Equity Fund \| Class R1
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVGJX
MFS Low Volatility Global Equity Fund \| Class R2
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVGKX
MFS Low Volatility Global Equity Fund \| Class R3
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVGLX
MFS Low Volatility Global Equity Fund \| Class R4
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVGMX
MFS Low Volatility Global Equity Fund \| Class R6
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVGNX
MFS New Discovery Fund \| Class A
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MNDAX
MFS New Discovery Fund \| Class B
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MNDBX
MFS New Discovery Fund \| Class C
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MNDCX
MFS New Discovery Fund \| Class I
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MNDIX
MFS New Discovery Fund \| Class R1
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MNDGX
MFS New Discovery Fund \| Class R2
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MNDRX
MFS New Discovery Fund \| Class R3
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MNDHX
MFS New Discovery Fund \| Class R4
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MNDJX
MFS New Discovery Fund \| Class R6
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MNDKX
MFS Research International Fund \| Class A
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRSAX
MFS Research International Fund \| Class B
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRIBX
MFS Research International Fund \| Class C
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRICX
MFS Research International Fund \| Class I
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRSIX
MFS Research International Fund \| Class R1
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRSGX
MFS Research International Fund \| Class R2
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRSRX
MFS Research International Fund \| Class R3
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRSHX
MFS Research International Fund \| Class R4
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRSJX
MFS Research International Fund \| Class R6
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MRSKX
MFS Technology Fund \| Class A
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MTCAX
MFS Technology Fund \| Class B
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MTCBX
MFS Technology Fund \| Class C
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MTCCX
MFS Technology Fund \| Class I
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MTCIX
MFS Technology Fund \| Class R1
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MTCKX
MFS Technology Fund \| Class R2
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MTERX
MFS Technology Fund \| Class R3
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MTCHX
MFS Technology Fund \| Class R4
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MTCJX
MFS Technology Fund \| Class R6
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MTCLX
MFS U.S. Government Cash Reserve Fund \| Class A
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MSRXX
MFS U.S. Government Cash Reserve Fund \| Class B
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MCRXX
MFS U.S. Government Cash Reserve Fund \| Class C
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MCCXX
MFS U.S. Government Cash Reserve Fund \| Class I
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	CRHXX
MFS U.S. Government Cash Reserve Fund \| Class R1
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	CRVXX
MFS U.S. Government Cash Reserve Fund \| Class R2
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	CRMXX
MFS U.S. Government Cash Reserve Fund \| Class R3
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	CRJXX
MFS U.S. Government Cash Reserve Fund \| Class R4
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	CRKXX
MFS U.S. Government Cash Reserve Fund \| Class R6
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	CRRXX
MFS Value Fund \| Class A
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MEIAX
MFS Value Fund \| Class B
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MFEBX
MFS Value Fund \| Class C
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MEICX
MFS Value Fund \| Class I
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MEIIX
MFS Value Fund \| Class R1
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MEIGX
MFS Value Fund \| Class R2
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MVRRX
MFS Value Fund \| Class R3
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MEIHX
MFS Value Fund \| Class R4
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MEIJX
MFS Value Fund \| Class R6
Risk/Return:	oef_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MEIKX

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- 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.

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No definition available.

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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.

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No definition available.

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- 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.

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- 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
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-Section 12
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+ References

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

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- 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

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- Definition

Trading symbol of an instrument as listed on an exchange.

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