Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	497
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 14, 2018
Entity Registrant Name	dei_EntityRegistrantName	MAINSTAY FUNDS
Entity Central Index Key	dei_EntityCentralIndexKey	0000787441
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Dec. 14, 2018
Document Effective Date	dei_DocumentEffectiveDate	Dec. 14, 2018
Prospectus Date	rr_ProspectusDate	Feb. 28, 2018
MainStay MacKay Government Fund (Prospectus Summary) \| MainStay MacKay Government Fund \| Class A
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MGVAX
MainStay MacKay Government Fund (Prospectus Summary) \| MainStay MacKay Government Fund \| Class B
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MCSGX
MainStay MacKay Government Fund (Prospectus Summary) \| MainStay MacKay Government Fund \| Class C
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MGVCX
MainStay MacKay Government Fund (Prospectus Summary) \| MainStay MacKay Government Fund \| Class I
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MGOIX
MainStay MacKay Government Fund (Prospectus Summary) \| MainStay MacKay Government Fund \| INVESTOR CLASS
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MGVNX
MainStay MacKay Government Fund (Prospectus Summary) \| MainStay MacKay Government Fund \| Class R6
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MGVDX
MainStay MacKay International Equity Fund (Prospectus Summary) \| MainStay MacKay International Equity Fund \| Class A
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MSEAX
MainStay MacKay International Equity Fund (Prospectus Summary) \| MainStay MacKay International Equity Fund \| INVESTOR CLASS
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MINNX
MainStay MacKay International Equity Fund (Prospectus Summary) \| MainStay MacKay International Equity Fund \| Class B
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MINEX
MainStay MacKay International Equity Fund (Prospectus Summary) \| MainStay MacKay International Equity Fund \| Class C
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MIECX
MainStay MacKay International Equity Fund (Prospectus Summary) \| MainStay MacKay International Equity Fund \| Class I
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MSIIX
MainStay MacKay International Equity Fund (Prospectus Summary) \| MainStay MacKay International Equity Fund \| Class R1
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MIERX
MainStay MacKay International Equity Fund (Prospectus Summary) \| MainStay MacKay International Equity Fund \| Class R2
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MIRRX
MainStay MacKay International Equity Fund (Prospectus Summary) \| MainStay MacKay International Equity Fund \| Class R3
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MIFRX
MainStay MacKay International Equity Fund (Prospectus Summary) \| MainStay MacKay International Equity Fund \| Class R6
Risk/Return:	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MIFDX

If the value is true, then the document is an amendment to previously-filed/accepted document.

No definition available.

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

- Definition

The date the document was made available and submitted, in CCYY-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

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Namespace Prefix:	dei_
Data Type:	xbrli:dateItemType
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Period Type:	duration

- Definition

The date when a document, upon receipt and acceptance, becomes officially effective, in CCYY-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

The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented. If there is no historical data in the report, use the filing date. The format of the date is CCYY-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 Regulation 12B
-Number 240
-Section 12b
-Subsection 1

+ Details

Name:	dei_EntityCentralIndexKey
Namespace Prefix:	dei_
Data Type:	dei:centralIndexKeyItemType
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 Regulation 12B
-Number 240
-Section 12b
-Subsection 1

+ 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:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The date of the prospectus.

+ References

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

+ Details

Name:	rr_ProspectusDate
Namespace Prefix:	rr_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	rr_RiskReturnAbstract
Namespace Prefix:	rr_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration