485BPOS

AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON OCTOBER 11, 2012

FILE NO.  333-160918

FILE NO.  811-22321

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER THE SECURITIES ACT OF 1933  [X]

Post-Effective Amendment No.  34

AND

REGISTRATION STATEMENT

UNDER THE INVESTMENT COMPANY ACT OF 1940  [X]

Amendment No.  36

MAINSTAY FUNDS TRUST

(exact name of registrant as specified in charter)

51 MADISON AVENUE, NEW YORK, NEW YORK 10010

(address of principal executive office)

REGISTRANT'S TELEPHONE NUMBER: (212) 576-7000

J. Kevin Gao, Esq. MainStay Funds Trust 169 Lackawanna Avenue Parsippany, NJ 07054 Copy to: Sander M. Bieber, Esq. Dechert LLP 1775 I Street, NW Washington, DC 20006

(NAME AND ADDRESS OF AGENT FOR SERVICE)

It is proposed that this filing will become effective

x Immediately upon filing pursuant to paragraph (b) of Rule 485

o on ________ pursuant to paragraph (b)(1) of Rule 485

o 60 days after filing pursuant to paragraph (a)(1) of Rule 485

o on ________, pursuant to paragraph (a)(1) of Rule 485

o 75 days after filing pursuant to paragraph (a)(2) of Rule 485

o on  ________  pursuant to paragraph (a)(2) of Rule 485

If appropriate, check the following box:

o This Post-Effective Amendment designates a new effective date for a previously filed post-effective amendment

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for effectiveness of this Registration Statement under Rule 485(b) under the Securities Act and that it has duly caused this Post-Effective Amendment No. 34 to its Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Parsippany in the State of New Jersey, on the 11^th day of October, 2012.

MAINSTAY FUNDS TRUST
By:	/s/ Stephen P. Fisher
	Stephen P. Fisher
	President and Principal Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment No. 34 to the Registration Statement has been signed below by the following persons in the capacities indicated on October 11, 2012.

SIGNATURE	TITLE
/s/ Stephen P. Fisher	President and Principal Executive Officer
Stephen P. Fisher
/s/ Susan B. Kerley*	Trustee and Chairman of the Board
Susan B. Kerley
/s/ John Y. Kim*	Trustee
John Y. Kim
/s/ Alan R. Latshaw*	Trustee
Alan R. Latshaw
/s/ Peter Meenan*	Trustee
Peter Meenan
/s/ Richard H. Nolan, Jr.*	Trustee
Richard H. Nolan, Jr.
/s/ Richard S. Trutanic*	Trustee
Richard S. Trutanic
/s/ Roman L. Weil*	Trustee
Roman L. Weil
/s/ John A. Weisser*	Trustee
John A. Weisser
/s/ Jack R. Benintende	Treasurer and Principal Financial
Jack R. Benintende	and Accounting Officer
*By: /s/ J. Kevin Gao
J. Kevin Gao
As Attorney-in-Fact

*    PURSUANT TO POWERS OF ATTORNEY PREVIOUSLY FILED

EXHIBIT INDEX

EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
ProspectusDate	rr_ProspectusDate	Oct. 05, 2012
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	MainStay Marketfield Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Fund seeks capital appreciation.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the MainStay Funds. This amount may vary depending on the MainStay Fund in which you invest. More information about these and other discounts is available from your financial professional and in the "Information on Sales Charges" section starting on page 23 of the Prospectus and in the "Alternative Sales Arrangements" section on page 62 of the Statement of Additional Information.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 138.22% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	138.22%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the MainStay Funds.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	50,000
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods (except as indicated with respect to Class C shares). The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same.
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	To achieve the Fund's investment objective, Marketfield Asset Management LLC, the Fund's Subadvisor, will allocate the Fund's assets among investments in equity securities, fixed-income securities and other investment companies, including exchange traded funds ("ETFs"), in proportions consistent with the Subadvisor's evaluation of their expected risks and returns. The Fund's equity securities investments may include common and preferred stocks of United States companies of any size. The Fund may invest up to 50% of its net assets in equity securities of foreign companies of any size, including up to 35% of its net assets in securities issued by corporations or governments located in developing or emerging markets. The Fund's investments in foreign securities may include, but are not limited to, American Depositary Receipts ("ADRs"), European Depositary Receipts ("EDRs") and Global Depositary Receipts ("GDRs"). Also, with respect to 50% of the Fund's net assets, the Fund may engage in short sales of securities to profit from an anticipated decline in the price of the securities sold short. Under normal market conditions, the Fund's investments in fixed-income securities consist of investment grade corporate bonds and debentures, mortgage-backed and asset-backed securities, United States Treasury obligations, municipal securities, obligations issued by the U.S. Government and its agencies or instrumentalities and convertible securities. However, the Fund may invest up to 30% of its net assets in fixed income securities that are below investment grade. Below investment grade securities are generally securities that receive low ratings from independent rating agencies, such as rated lower than BBB- by Standard & Poor's ("S&P") and Baa3 by Moody's Investor Service, Inc. ("Moody's"), or if unrated, are determined to be of equivalent quality by the Subadvisor. If independent rating agencies assign different ratings to the same security, the Fund will use the higher rating for purposes of determining the security's credit quality. Securities that are rated below investment grade by independent rating agencies are commonly referred to as "high yield debt" or "junk bonds. " The fixed-income securities in which the Fund invests may have maturities of any length and may have variable and floating interest rates. The Fund may also invest in zero-coupon bonds, without limitation. In addition, the Fund may invest up to 50% of its net assets in equity or fixed-income options, futures contracts and convertible securities and may invest up to 30% of its net assets in swap agreements. The Subadvisor shall manage the Fund so that the Fund will not be deemed to be a "commodity pool operator" under the Commodity Exchange Act. Investment Process. When reviewing investment opportunities for the Fund, the Subadvisor considers various factors, including macroeconomic conditions, corporate earnings at a macroeconomic level, anticipated inflation and interest rates, consumer risk and its perception of the outlook of the capital markets as a whole. A macroeconomic strategy focuses on broad trends and is generally distinguished from a strategy that focuses on the prospects of particular companies or issuers. The Subadvisor may allocate the Fund's investments between equity and fixed-income securities at its discretion, without limitation. Security selection for the Fund is driven by the Subadvisor's top-down analysis of economic issues, the Subadvisor's perception of investor sentiment and investment flows. Once the Subadvisor has identified a theme that is expected to either benefit or disadvantage a specific sector or country, it seeks to implement an investment strategy that is appropriate for the Fund. In some cases, the Subadvisor may utilize a sector- or country-specific ETF that offers exposure to a broad range of securities. In other situations, the Subadvisor may select a single issue that is perceived by the Subadvisor to be particularly germane to a specific concern or a small group of issues with characteristics that match the goal of creating portfolio exposure to a macroeconomic theme. The Subadvisor may sell a security if it no longer believes the security will contribute to meeting the investment objective of the Fund or when the security is deemed less attractive relative to another security on a return/risk basis. The Subadvisor may also sell or reduce a position in a security if it sees the investment theme failing to materialize.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Loss of Money Risk: Before considering an investment in the Fund, you should understand that you could lose money. Market Changes Risk: The value of the Fund's investments may change because of broad changes in the markets in which the Fund invests, which could cause the Fund to underperform other funds with similar objectives. From time to time, markets may experience periods of acute stress that may result in increased volatility. Such market conditions tend to add significantly to the risk of short-term volatility in the net asset value of the Fund's shares. Management Risk: The investment strategies, practices and risk analysis used by the Fund's Subadvisor may not produce the desired results. Equity Securities Risk: Investments in common stocks and other equity securities are particularly subject to the risk of changing economic, stock market, industry and company conditions and the risks inherent in the portfolio manager's ability to anticipate such changes that can adversely affect the value of the Fund's holdings. Opportunity for greater gain often comes with greater risk of loss. Convertible Securities Risk: Convertible securities may be subordinate to other securities. In part, the total return for a convertible security depends upon performance of the underlying stock into which it can be converted. Also, issuers of convertible securities are often not as strong financially as those issuing securities with higher credit ratings, are more likely to encounter financial difficulties and typically are more vulnerable to changes in the economy, such as a recession or a sustained period of rising interest rates, which could affect their ability to make interest and principal payments. If an issuer stops making interest and/or principal payments, the Fund could lose its entire investment. Foreign Securities Risk: Investments in foreign securities may be riskier than investments in U.S. securities. Differences between U.S. and foreign regulatory regimes and securities markets, including less stringent investor protections and disclosure standards of some foreign markets, less liquid trading markets, as well as political and economic developments in foreign countries, may affect the value of the Fund's investments in foreign securities. Foreign securities may also subject the Fund's investments to changes in currency rates. Emerging Markets Risk: The risks related to investing in foreign securities are generally greater with respect to securities of companies that conduct their business activities in emerging markets or whose securities are traded principally in emerging markets. The risks of investing in emerging markets include the risks of illiquidity, increased price volatility, smaller market capitalizations, less government regulation, less extensive and less frequent accounting, financial and other reporting requirements, risk of loss resulting from problems in share registration and custody, substantial economic and political disruptions and the nationalization of foreign deposits or assets. Mid-Cap Stock Risk: Stocks of mid-cap companies may be subject to greater price volatility, significantly lower trading volumes, cyclical, static or moderate growth prospects and greater spreads between their bid and ask prices than stocks of larger companies. Because these businesses frequently rely on narrower product lines and niche markets, they can suffer isolated setbacks. Small-Cap Stock Risk: Stocks of small capitalization companies may be subject to greater price volatility, significantly lower trading volumes, cyclical, static or moderate growth prospects, and greater spreads between bid and ask prices than stocks of larger companies. Small-capitalization companies may be more vulnerable to adverse business or market developments. Short Selling Risk: If a security sold short increases in price, the Fund may have to cover its short position at a higher price than the short sale price, resulting in a loss. The Fund may have substantial short positions and must borrow those securities to make delivery to the buyer. The Fund may not be able to borrow a security that it needs to deliver or it may not be able to close out a short position at an acceptable price and may have to sell related long positions before it had intended to do so. Thus, the Fund may not be able to successfully implement its short sale strategy due to limited availability of desired securities or for other reasons. The Fund also may be required to pay a premium and other transaction costs, which would increase the cost of the security sold short. The amount of any gain will be decreased, and the amount of any loss increased, by the amount of the premium, dividends, interest or expenses the Fund may be required to pay in connection with the short sale. Until the Fund replaces a borrowed security, it is required to maintain a segregated account of cash or liquid assets with the Fund's custodian to cover the Fund's short position. Generally, securities held in a segregated account cannot be sold unless they are replaced with other liquid assets. The Fund's ability to access the pledged collateral may also be impaired in the event the broker fails to comply with the terms of the contract. In such instances the Fund may not be able to substitute or sell the pledged collateral. Additionally, the Fund must maintain sufficient liquid assets (less any additional collateral pledged to the broker), marked-to-market daily, to cover the short sale obligations. This may limit the Fund's investment flexibility, as well as its ability to meet redemption requests or other current obligations. Because losses on short sales arise from increases in the value of the security sold short, such losses are theoretically unlimited. By contrast, a loss on a long position arises from decreases in the value of the security and is limited by the fact that a security's value cannot go below zero. By investing the proceeds received from selling securities short, the Fund could be deemed to be employing a form of leverage, which creates special risks. The use of leverage may increase the Fund's exposure to long positions and make any change in the Fund's net asset value greater than it would be without the use of leverage. This could result in increased volatility of returns. There is no guarantee that the Fund will leverage its portfolio, or if it does, that the Fund's leveraging strategy will be successful or that it will produce a higher return on an investment. Regulatory authorities in the U.S. or other countries may prohibit or restrict the ability of the Fund to fully implement its short-selling strategy, either generally or with respect to certain industries or countries, which may impact the Fund's ability to fully implement its investment strategies. Depositary Receipts Risk: Investments in depositary receipts may entail the special risks of foreign investing, including currency exchange fluctuations, government regulations, and the potential for political and economic instability. Debt Securities Risk: The risks of investing in debt securities include (without limitation): (i) credit risk, i.e., the issuer may not repay the loan created by the issuance of that debt security; (ii) maturity risk, i.e., a debt security with a longer maturity may fluctuate in value more than one with a shorter maturity; (iii) market risk, i.e., low demand of debt securities may negatively impact their price; (iv) interest rate risk, i.e., when interest rates go up, the value of a debt security goes down, and when interest rates go down, the value of a debt security goes up; (v) selection risk, i.e., the securities selected by the Subadvisor may underperform the market or other securities selected by other funds; and (vi) call risk, i.e., during a period of falling interest rates, the issuer may redeem a security by repaying it early, which may reduce the Fund's income, if the proceeds are reinvested at lower interest rates. Additional risks associated with an investment in the Fund include the following: (i) not all U.S. government securities are insured or guaranteed by the U.S. government--some are backed only by the issuing agency, which must rely on its own resources to repay the debt; and (ii) the Fund's yield will fluctuate with changes in short-term interest rates. High-Yield Securities Risk: Investments in high-yield securities (commonly referred to as "junk bonds") are sometimes considered speculative because they present a greater risk of loss than higher quality securities. Such securities may, under certain circumstances, be less liquid than higher rated securities. These securities pay investors a premium (a high interest rate or yield) because of the increased risk of loss. These securities can also be subject to greater price volatility. In times of unusual or adverse market, economic or political conditions, these securities may experience higher than normal default rates. Floaters and Variable Rate Notes Risk: Floaters and variable rate notes provide for a periodic adjustment in the interest rate paid on the securities. The rate adjustment intervals may be regular and range from daily up to annually, or may be based on an event, such as a change in the prime rate. Floating and variable rate notes may be subject to greater liquidity risk than other debt securities, meaning that there may be limitations on the Fund's ability to sell the securities at any given time. Securities with floating interest rates generally are less sensitive to interest rate changes, but may decline in value if their interest rates do not rise as much or as fast as interest rates in general. Such securities also may lose value. Other Investment Companies Risk: The Fund may invest in other investment companies, including open-end funds, closed-end funds, and ETFs. The Fund may purchase the securities of another investment company to temporarily gain exposure to a portion of the market while awaiting purchase of securities or as an efficient means of gaining exposure to a particular asset class. The Fund might also purchase shares of another investment company to gain exposure to the securities in the investment company's portfolio at times when the Fund may not be able to buy those securities directly. Any investment in another investment company would be consistent with the Fund's objective and investment program. The risks of owning another investment company are generally similar to the risks of investment directly in the securities in which that investment company invests. However, an investment company may not achieve its investment objective or execute its investment strategy effectively, which may adversely affect the Fund's performance. In addition, because closed-end funds and ETFs trade on a secondary market, their shares may trade at a premium or discount to the actual net asset value of their portfolio securities and their shares may have greater volatility because of the potential lack of liquidity. Exchange Traded Fund Risk: The risks of owning an ETF generally reflect the risks of owning the underlying securities they are designed to track, although lack of liquidity in an ETF could result in it being more volatile than the underlying portfolio of securities. Disruptions in the markets for the securities underlying ETFs purchased or sold by the Fund could result in losses on the Fund's investment in ETFs. ETFs also have management fees that increase their costs versus the costs of owning the underlying securities directly. Zero Coupon Bond Risk: Because zero-coupon securities bear no interest and compound semi-annually at the rate fixed at the time of issuance, their value generally is more volatile than the value of other fixed income securities. An investment in zero-coupon and delayed interest securities may cause the Fund to recognize income, and therefore the Fund may be required to make distributions to shareholders before the Fund receives any cash payments on its investment. Mortgage-Backed/Asset-Backed Securities Risk: Prepayment risk is associated with mortgage-backed and asset-backed securities. If interest rates fall, the underlying debt may be repaid ahead of schedule, reducing the value of the Fund's investments. If interest rates rise, there may be fewer prepayments, which would cause the average bond maturity to rise, increasing the potential for the Fund to lose money. The value of these securities may be significantly affected by changes in interest rates, the market's perception of issuers, and the creditworthiness of the parties involved. The ability of the Fund to successfully utilize these instruments may depend on the ability of the Subadvisor to forecast interest rates and other economic factors correctly. These securities may have a structure that makes their reaction to interest rate changes and other factors difficult to predict, making their value highly volatile. Municipal Bond Risk: Municipal bond risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers, and the possibility of future tax and legislative changes, which could affect the market for and value of municipal securities. Municipalities continue to experience economic and financial difficulties in the current economic environment. The ability of a municipal issuer to make payments and the value of municipal bonds can be affected by uncertainties in the municipal securities market. Such uncertainties could cause increased volatility in the municipal securities market and could negatively impact the Fund's net asset value. Derivatives Risk: Derivatives are investments whose value depends on (or is derived from) the value of an underlying instrument, such as a security, asset, reference rate or index. Derivative strategies often involve leverage, which may exaggerate a loss, potentially causing the Fund to lose more money than it would have lost had it invested in the underlying instrument. Derivatives may be difficult to sell, unwind or value. Derivatives may also be subject to counterparty risk, which is the risk that the counterparty (the party on the other side of the transaction) on a derivative transaction will be unable to honor its contractual obligations to the Fund. Swap transactions tend to shift the Fund's investment exposure from one type of investment to another, and therefore entail the risk that a party will default on its payment obligations to the Fund. Futures may be more volatile than direct investments in the instrument underlying the futures, and may not correlate perfectly to the underlying instrument. Futures also may involve a small initial investment relative to the risk assumed, which could result in losses greater than if they had not been used. Due to fluctuations in the price of the underlying security, the Fund may not be able to profitably exercise an option and may lose its entire investment in an option. Forward commitments entail the risk that the instrument may be worth less when it is issued or received than the price the Fund agreed to pay when it made the commitment. The use of foreign currency forwards may result in currency exchange losses due to fluctuations in currency exchange rates or an imperfect correlation between portfolio holdings denominated in a particular currency and the forward contracts entered into by the Fund. Tax Risk: The Fund's investments and investment strategies, including transactions in options and futures contracts, may be subject to special and complex federal income tax provisions, the effect of which may be, among other things: (1) to disallow, suspend, defer or otherwise limit the allowance of certain losses or deductions; (2) to accelerate income to the Fund; (3) to convert long-term capital gain, which is currently subject to lower tax rates, into short-term capital gain or ordinary income, which are currently subject to higher tax rates; (4) to convert an ordinary loss or a deduction into a capital loss (the deductibility of which is more limited); (5) to treat dividends that would otherwise constitute qualified dividend income as non-qualified dividend income; (6) to produce income that will not qualify as good income under the gross income requirements that must be met for the Fund to qualify as a regulated investment company (a "RIC") under Subchapter M of the Internal Revenue Code of 1986, as amended (the "Code"). Furthermore, to the extent that any futures contract or option on a futures contract held by the Fund is a "section 1256 contract" under Section 1256 of the Code, the contract will be marked to market annually, and any gain or loss will be treated as 60% long-term and 40% short-term, regardless of the holding period for such contract. Section 1256 contracts include Fund transactions involving call options on a broad-based securities index, certain futures contracts and other financial contracts.
Risk Lose Money [Text]	rr_RiskLoseMoney	Before considering an investment in the Fund, you should understand that you could lose money.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Past Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following bar chart and tables indicate some of the risks of investing in the Fund. The bar chart shows you how the Fund's calendar year performance has varied over the life of the Fund. Sales loads are not reflected in the bar chart. If they were, returns would be less than those shown. The average annual total returns table shows how the Fund's average annual total returns (before and after taxes) for the one-year period and life of the Fund compare to those of a broad-based securities market index. The Fund has selected the S&P 500®Index as its primary benchmark. The S&P 500® Index is widely regarded as the standard index for measuring large-cap U.S. stock market performance. The performance figures for Class I shares reflect the historical performance of the then-existing shares of Marketfield Fund (the predecessor to the Fund, which was subject to a different fee structure, and for which a predecessor entity to Marketfield Asset Management LLC served as investment advisor) for periods prior to October 5, 2012. Performance figures for all other share classes reflect the historical performance of Class I shares and are adjusted to reflect differences in fees and expenses. Performance data for the classes varies based on differences in their fee and expense structures. The Fund commenced operations on July 31, 2007. Unadjusted, the performance for the newer classes might have been different. Past performance of Marketfield Fund (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Please visit mainstayinvestments.com for more recent performance information.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following bar chart and tables indicate some of the risks of investing in the Fund.
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	mainstayinvestments.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	Past performance of Marketfield Fund (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Annual Returns, Class I Shares (by calendar year 2008-2011)
Bar Chart Narrative [Text Block]	rr_BarChartNarrativeTextBlock	The bar chart shows you how the Fund's calendar year performance has varied over the life of the Fund. Sales loads are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales loads are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter     2Q/09    15.89% Worst Quarter    4Q/08   -13.05% As of June 30, 2012, the Class I shares of the Fund had a year-to-date return of 8.77%.
Index No Deduction for Fees, Expenses, Taxes [Text]	rr_IndexNoDeductionForFeesExpensesTaxes	reflects no deductions for fees, expenses, or taxes
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns are not relevant if you hold your shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-tax returns shown are for Class I shares. After-tax returns for the other share classes may vary.
Performance Table Explanation after Tax Higher	rr_PerformanceTableExplanationAfterTaxHigher	In some cases, the return after taxes may exceed the return before taxes due to an assumed tax benefit from any losses on a sale of shares at the end of the measurement period.
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. In some cases, the return after taxes may exceed the return before taxes due to an assumed tax benefit from any losses on a sale of shares at the end of the measurement period. Actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns are not relevant if you hold your shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns shown are for Class I shares. After-tax returns for the other share classes may vary.
Caption	rr_AverageAnnualReturnCaption	Average Annual Total Returns (for the periods ended December 31, 2011)
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | S&P 500®Index
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	S&P 500® Index (reflects no deductions for fees, expenses, or taxes)
1 Year	rr_AverageAnnualReturnYear01	2.11%
Since Inception	rr_AverageAnnualReturnSinceInception	(1.09%)
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Investor Class
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.50%
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	1.40%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Dividend Expense on Securities Sold Short	rr_Component1OtherExpensesOverAssets	0.66%
Broker Fees and Charges on Short Sales	rr_Component2OtherExpensesOverAssets	0.21%
Remainder of Other Expenses	rr_Component3OtherExpensesOverAssets	0.30%
Total Other Expenses	rr_OtherExpensesOverAssets	1.17%
Acquired (Underlying) Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.04%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	2.86%	[2]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	823
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	1,387
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,976
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	3,560
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Investor Class
1 Year	rr_AverageAnnualReturnYear01	(2.25%)
Since Inception	rr_AverageAnnualReturnSinceInception	6.36%
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class A
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.50%
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	1.40%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Dividend Expense on Securities Sold Short	rr_Component1OtherExpensesOverAssets	0.66%
Broker Fees and Charges on Short Sales	rr_Component2OtherExpensesOverAssets	0.21%
Remainder of Other Expenses	rr_Component3OtherExpensesOverAssets	0.14%
Total Other Expenses	rr_OtherExpensesOverAssets	1.01%
Acquired (Underlying) Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.04%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	2.70%	[2]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	808
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	1,342
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,901
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	3,415
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Class A
1 Year	rr_AverageAnnualReturnYear01	(2.25%)
Since Inception	rr_AverageAnnualReturnSinceInception	6.36%
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class C
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	1.00%
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	1.40%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Dividend Expense on Securities Sold Short	rr_Component1OtherExpensesOverAssets	0.66%
Broker Fees and Charges on Short Sales	rr_Component2OtherExpensesOverAssets	0.21%
Remainder of Other Expenses	rr_Component3OtherExpensesOverAssets	0.30%
Total Other Expenses	rr_OtherExpensesOverAssets	1.17%
Acquired (Underlying) Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.04%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	3.61%	[2]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	464
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	1,106
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,869
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	3,871
Expense Example, No Redemption, 1 Year	rr_ExpenseExampleNoRedemptionYear01	364
Expense Example, No Redemption, 3 Years	rr_ExpenseExampleNoRedemptionYear03	1,106
Expense Example, No Redemption, 5 Years	rr_ExpenseExampleNoRedemptionYear05	1,869
Expense Example, No Redemption, 10 Years	rr_ExpenseExampleNoRedemptionYear10	3,871
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Class C
1 Year	rr_AverageAnnualReturnYear01	1.67%
Since Inception	rr_AverageAnnualReturnSinceInception	6.93%
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class I
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	1.40%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Dividend Expense on Securities Sold Short	rr_Component1OtherExpensesOverAssets	0.66%
Broker Fees and Charges on Short Sales	rr_Component2OtherExpensesOverAssets	0.21%
Remainder of Other Expenses	rr_Component3OtherExpensesOverAssets	0.14%
Total Other Expenses	rr_OtherExpensesOverAssets	1.01%
Acquired (Underlying) Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.04%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	2.45%	[2]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	248
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	764
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,306
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	2,786
Annual Return 2008	rr_AnnualReturn2008	(12.88%)
Annual Return 2009	rr_AnnualReturn2009	31.08%
Annual Return 2010	rr_AnnualReturn2010	14.32%
Annual Return 2011	rr_AnnualReturn2011	3.70%
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Jun. 30, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	8.77%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	15.89%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(13.05%)
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Class I
1 Year	rr_AverageAnnualReturnYear01	3.70%
Since Inception	rr_AverageAnnualReturnSinceInception	8.00%
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class I | After Taxes on Distributions
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions Class I
1 Year	rr_AverageAnnualReturnYear01	3.70%
Since Inception	rr_AverageAnnualReturnSinceInception	7.98%
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class I | After Taxes on Distributions and Sales
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions and Sale of Fund Shares Class I
1 Year	rr_AverageAnnualReturnYear01	2.40%
Since Inception	rr_AverageAnnualReturnSinceInception	6.92%
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class R2
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	1.40%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Dividend Expense on Securities Sold Short	rr_Component1OtherExpensesOverAssets	0.66%
Broker Fees and Charges on Short Sales	rr_Component2OtherExpensesOverAssets	0.21%
Remainder of Other Expenses	rr_Component3OtherExpensesOverAssets	0.24%
Total Other Expenses	rr_OtherExpensesOverAssets	1.11%
Acquired (Underlying) Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.04%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	2.80%	[2]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	283
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	868
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,479
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	3,128
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Class R2
1 Year	rr_AverageAnnualReturnYear01	3.34%
Since Inception	rr_AverageAnnualReturnSinceInception	7.62%

[1] A contingent deferred sales charge of 1.00% may be imposed on certain redemptions made within one year of the date of purchase on shares that were purchased without an initial sales charge.

[2] Effective October 5, 2012, New York Life Investment Management LLC ("New York Life Investments") has contractually agreed to waive fees and/or reimburse expenses so that Total Annual Fund Operating Expenses (excluding taxes, interest, litigation, extraordinary expenses, brokerage and other transaction expenses relating to the purchase or sale of portfolio investments, and acquired (underlying) fund fees and expenses) for Class I shares do not exceed 1.56% of its average daily net assets. This agreement will be in effect for a two-year period unless extended by New York Life Investments and approved by the Board of Trustees.

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MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund

MainStay Marketfield Fund

Investment Objective

The Fund seeks capital appreciation.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you may pay if you buy and
hold shares of the Fund. You may qualify for sales charge discounts if you and
your family invest, or agree to invest in the future, at least $50,000 in the
MainStay Funds. This amount may vary depending on the MainStay Fund in which you
invest. More information about these and other discounts is available from your
financial professional and in the "Information on Sales Charges" section starting
on page 23 of the Prospectus and in the "Alternative Sales Arrangements" section
on page 62 of the Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees MainStay Marketfield Fund	Investor Class		Class A		Class C	Class I	Class R2
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	5.50%		5.50%		none	none	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	none	[1]	none	[1]	1.00%	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses MainStay Marketfield Fund		Investor Class	Class A	Class C	Class I	Class R2
Management Fees (as an annual percentage of the Fund's average daily net assets)		1.40%	1.40%	1.40%	1.40%	1.40%
Distribution and/or Service (12b-1) Fees		0.25%	0.25%	1.00%	none	0.25%
Dividend Expense on Securities Sold Short		0.66%	0.66%	0.66%	0.66%	0.66%
Broker Fees and Charges on Short Sales		0.21%	0.21%	0.21%	0.21%	0.21%
Remainder of Other Expenses		0.30%	0.14%	0.30%	0.14%	0.24%
Total Other Expenses		1.17%	1.01%	1.17%	1.01%	1.11%
Acquired (Underlying) Fund Fees and Expenses		0.04%	0.04%	0.04%	0.04%	0.04%
Total Annual Fund Operating Expenses	[1]	2.86%	2.70%	3.61%	2.45%	2.80%

Example

The Example is intended to help you compare the cost of investing in the Fund
with the cost of investing in other mutual funds. The Example assumes that you
invest $10,000 in the Fund for the time periods indicated and then redeem all
of your shares at the end of those periods (except as indicated with respect to
Class C shares). The Example also assumes that your investment has a 5% return
each year and that the Fund's operating expenses remain the same.

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example MainStay Marketfield Fund (USD $)	Investor Class	Class A	Class C	Class I	Class R2
Expense Example, with Redemption, 1 Year	823	808	464	248	283
Expense Example, with Redemption, 3 Years	1,387	1,342	1,106	764	868
Expense Example, with Redemption, 5 Years	1,976	1,901	1,869	1,306	1,479
Expense Example, with Redemption, 10 Years	3,560	3,415	3,871	2,786	3,128

Expense Example, No Redemption (USD $)	Expense Example, No Redemption, 1 Year	Expense Example, No Redemption, 3 Years	Expense Example, No Redemption, 5 Years	Expense Example, No Redemption, 10 Years
MainStay Marketfield Fund Class C	364	1,106	1,869	3,871

Portfolio Turnover

The Fund pays transaction costs, such as commissions, when it buys and sells
securities (or "turns over" its portfolio). A higher portfolio turnover may
indicate higher transaction costs and may result in higher taxes when Fund
shares are held in a taxable account. These costs, which are not reflected in
annual fund operating expenses or in the Example, affect the Fund's performance.
During the most recent fiscal year, the Fund's portfolio turnover rate was
138.22% of the average value of its portfolio.

Principal Investment Strategies

To achieve the Fund's investment objective, Marketfield Asset Management LLC,
the Fund's Subadvisor, will allocate the Fund's assets among investments in
equity securities, fixed-income securities and other investment companies,
including exchange traded funds ("ETFs"), in proportions consistent with the
Subadvisor's evaluation of their expected risks and returns.

The Fund's equity securities investments may include common and preferred stocks
of United States companies of any size. The Fund may invest up to 50% of its net
assets in equity securities of foreign companies of any size, including up to
35% of its net assets in securities issued by corporations or governments
located in developing or emerging markets. The Fund's investments in foreign
securities may include, but are not limited to, American Depositary Receipts
("ADRs"), European Depositary Receipts ("EDRs") and Global Depositary Receipts
("GDRs"). Also, with respect to 50% of the Fund's net assets, the Fund may engage
in short sales of securities to profit from an anticipated decline in the price
of the securities sold short.

Under normal market conditions, the Fund's investments in fixed-income securities
consist of investment grade corporate bonds and debentures, mortgage-backed and
asset-backed securities, United States Treasury obligations, municipal securities,
obligations issued by the U.S. Government and its agencies or instrumentalities
and convertible securities. However, the Fund may invest up to 30% of its net
assets in fixed income securities that are below investment grade. Below
investment grade securities are generally securities that receive low ratings
from independent rating agencies, such as rated lower than BBB- by Standard
& Poor's ("S&P") and Baa3 by Moody's Investor Service, Inc. ("Moody's"), or
if unrated, are determined to be of equivalent quality by the Subadvisor. If
independent rating agencies assign different ratings to the same security,
the Fund will use the higher rating for purposes of determining the security's
credit quality. Securities that are rated below investment grade by independent
rating agencies are commonly referred to as "high yield debt" or "junk bonds.
" The fixed-income securities in which the Fund invests may have maturities of
any length and may have variable and floating interest rates. The Fund may also
invest in zero-coupon bonds, without limitation.

In addition, the Fund may invest up to 50% of its net assets in equity or
fixed-income options, futures contracts and convertible securities and may
invest up to 30% of its net assets in swap agreements. The Subadvisor shall
manage the Fund so that the Fund will not be deemed to be a "commodity pool
operator" under the Commodity Exchange Act.

Investment Process. When reviewing investment opportunities for the Fund,
the Subadvisor considers various factors, including macroeconomic conditions,
corporate earnings at a macroeconomic level, anticipated inflation and interest
rates, consumer risk and its perception of the outlook of the capital markets as
a whole. A macroeconomic strategy focuses on broad trends and is generally
distinguished from a strategy that focuses on the prospects of particular companies
or issuers. The Subadvisor may allocate the Fund's investments between equity and
fixed-income securities at its discretion, without limitation.

Security selection for the Fund is driven by the Subadvisor's top-down analysis
of economic issues, the Subadvisor's perception of investor sentiment and
investment flows. Once the Subadvisor has identified a theme that is expected
to either benefit or disadvantage a specific sector or country, it seeks to
implement an investment strategy that is appropriate for the Fund. In some
cases, the Subadvisor may utilize a sector- or country-specific ETF that offers
exposure to a broad range of securities. In other situations, the Subadvisor may
select a single issue that is perceived by the Subadvisor to be particularly
germane to a specific concern or a small group of issues with characteristics
that match the goal of creating portfolio exposure to a macroeconomic theme.

The Subadvisor may sell a security if it no longer believes the security will
contribute to meeting the investment objective of the Fund or when the security
is deemed less attractive relative to another security on a return/risk basis.
The Subadvisor may also sell or reduce a position in a security if it sees the
investment theme failing to materialize.

Principal Risks

Loss of Money Risk: Before considering an investment in the Fund, you should
understand that you could lose money.

Market Changes Risk: The value of the Fund's investments may change because of
broad changes in the markets in which the Fund invests, which could cause the
Fund to underperform other funds with similar objectives. From time to time,
markets may experience periods of acute stress that may result in increased
volatility. Such market conditions tend to add significantly to the risk of
short-term volatility in the net asset value of the Fund's shares.

Management Risk: The investment strategies, practices and risk analysis used by
the Fund's Subadvisor may not produce the desired results.

Equity Securities Risk: Investments in common stocks and other equity securities
are particularly subject to the risk of changing economic, stock market, industry
and company conditions and the risks inherent in the portfolio manager's ability
to anticipate such changes that can adversely affect the value of the Fund's
holdings. Opportunity for greater gain often comes with greater risk of loss.

Convertible Securities Risk: Convertible securities may be subordinate to other
securities. In part, the total return for a convertible security depends upon
performance of the underlying stock into which it can be converted. Also, issuers
of convertible securities are often not as strong financially as those issuing
securities with higher credit ratings, are more likely to encounter financial
difficulties and typically are more vulnerable to changes in the economy, such
as a recession or a sustained period of rising interest rates, which could affect
their ability to make interest and principal payments. If an issuer stops making
interest and/or principal payments, the Fund could lose its entire investment.

Foreign Securities Risk: Investments in foreign securities may be riskier than
investments in U.S. securities. Differences between U.S. and foreign regulatory
regimes and securities markets, including less stringent investor protections
and disclosure standards of some foreign markets, less liquid trading markets,
as well as political and economic developments in foreign countries, may affect
the value of the Fund's investments in foreign securities. Foreign securities
may also subject the Fund's investments to changes in currency rates.

Emerging Markets Risk: The risks related to investing in foreign securities
are generally greater with respect to securities of companies that conduct
their business activities in emerging markets or whose securities are traded
principally in emerging markets. The risks of investing in emerging markets
include the risks of illiquidity, increased price volatility, smaller market
capitalizations, less government regulation, less extensive and less frequent
accounting, financial and other reporting requirements, risk of loss resulting
from problems in share registration and custody, substantial economic and
political disruptions and the nationalization of foreign deposits or assets.

Mid-Cap Stock Risk: Stocks of mid-cap companies may be subject to greater price
volatility, significantly lower trading volumes, cyclical, static or moderate
growth prospects and greater spreads between their bid and ask prices than
stocks of larger companies. Because these businesses frequently rely on narrower
product lines and niche markets, they can suffer isolated setbacks.

Small-Cap Stock Risk: Stocks of small capitalization companies may be subject to
greater price volatility, significantly lower trading volumes, cyclical, static
or moderate growth prospects, and greater spreads between bid and ask prices
than stocks of larger companies. Small-capitalization companies may be more
vulnerable to adverse business or market developments.

Short Selling Risk: If a security sold short increases in price, the Fund may
have to cover its short position at a higher price than the short sale price,
resulting in a loss. The Fund may have substantial short positions and must
borrow those securities to make delivery to the buyer. The Fund may not be
able to borrow a security that it needs to deliver or it may not be able to
close out a short position at an acceptable price and may have to sell related
long positions before it had intended to do so. Thus, the Fund may not be able
to successfully implement its short sale strategy due to limited availability of
desired securities or for other reasons.

The Fund also may be required to pay a premium and other transaction costs, which
would increase the cost of the security sold short. The amount of any gain will
be decreased, and the amount of any loss increased, by the amount of the premium,
dividends, interest or expenses the Fund may be required to pay in connection with
the short sale.

Until the Fund replaces a borrowed security, it is required to maintain a
segregated account of cash or liquid assets with the Fund's custodian to cover
the Fund's short position. Generally, securities held in a segregated account
cannot be sold unless they are replaced with other liquid assets. The Fund's
ability to access the pledged collateral may also be impaired in the event the
broker fails to comply with the terms of the contract. In such instances the
Fund may not be able to substitute or sell the pledged collateral. Additionally,
the Fund must maintain sufficient liquid assets (less any additional collateral
pledged to the broker), marked-to-market daily, to cover the short sale obligations.
This may limit the Fund's investment flexibility, as well as its ability to meet
redemption requests or other current obligations.

Because losses on short sales arise from increases in the value of the security
sold short, such losses are theoretically unlimited. By contrast, a loss on a
long position arises from decreases in the value of the security and is limited
by the fact that a security's value cannot go below zero.

By investing the proceeds received from selling securities short, the Fund could
be deemed to be employing a form of leverage, which creates special risks. The
use of leverage may increase the Fund's exposure to long positions and make any
change in the Fund's net asset value greater than it would be without the use
of leverage. This could result in increased volatility of returns. There is no
guarantee that the Fund will leverage its portfolio, or if it does, that the
Fund's leveraging strategy will be successful or that it will produce a higher
return on an investment.

Regulatory authorities in the U.S. or other countries may prohibit or restrict
the ability of the Fund to fully implement its short-selling strategy, either
generally or with respect to certain industries or countries, which may impact
the Fund's ability to fully implement its investment strategies.

Depositary Receipts Risk: Investments in depositary receipts may entail the
special risks of foreign investing, including currency exchange fluctuations,
government regulations, and the potential for political and economic
instability.

Debt Securities Risk: The risks of investing in debt securities include (without
limitation): (i) credit risk, i.e., the issuer may not repay the loan created by
the issuance of that debt security; (ii) maturity risk, i.e., a debt security
with a longer maturity may fluctuate in value more than one with a shorter
maturity; (iii) market risk, i.e., low demand of debt securities may negatively
impact their price; (iv) interest rate risk, i.e., when interest rates go up,
the value of a debt security goes down, and when interest rates go down, the
value of a debt security goes up; (v) selection risk, i.e., the securities
selected by the Subadvisor may underperform the market or other securities
selected by other funds; and (vi) call risk, i.e., during a period of falling
interest rates, the issuer may redeem a security by repaying it early, which
may reduce the Fund's income, if the proceeds are reinvested at lower interest
rates.

Additional risks associated with an investment in the Fund include the
following: (i) not all U.S. government securities are insured or guaranteed by
the U.S. government--some are backed only by the issuing agency, which must rely
on its own resources to repay the debt; and (ii) the Fund's yield will fluctuate
with changes in short-term interest rates.

High-Yield Securities Risk: Investments in high-yield securities (commonly
referred to as "junk bonds") are sometimes considered speculative because they
present a greater risk of loss than higher quality securities. Such securities
may, under certain circumstances, be less liquid than higher rated securities.
These securities pay investors a premium (a high interest rate or yield) because
of the increased risk of loss. These securities can also be subject to greater
price volatility. In times of unusual or adverse market, economic or political
conditions, these securities may experience higher than normal default rates.

Floaters and Variable Rate Notes Risk: Floaters and variable rate notes provide
for a periodic adjustment in the interest rate paid on the securities. The rate
adjustment intervals may be regular and range from daily up to annually, or may
be based on an event, such as a change in the prime rate. Floating and variable
rate notes may be subject to greater liquidity risk than other debt securities,
meaning that there may be limitations on the Fund's ability to sell the
securities at any given time. Securities with floating interest rates generally
are less sensitive to interest rate changes, but may decline in value if their
interest rates do not rise as much or as fast as interest rates in general. Such
securities also may lose value.

Other Investment Companies Risk: The Fund may invest in other investment
companies, including open-end funds, closed-end funds, and ETFs.

The Fund may purchase the securities of another investment company to temporarily
gain exposure to a portion of the market while awaiting purchase of securities or
as an efficient means of gaining exposure to a particular asset class. The Fund
might also purchase shares of another investment company to gain exposure to the
securities in the investment company's portfolio at times when the Fund may not
be able to buy those securities directly. Any investment in another investment
company would be consistent with the Fund's objective and investment program.

The risks of owning another investment company are generally similar to the
risks of investment directly in the securities in which that investment company
invests. However, an investment company may not achieve its investment objective
or execute its investment strategy effectively, which may adversely affect the
Fund's performance. In addition, because closed-end funds and ETFs trade on a
secondary market, their shares may trade at a premium or discount to the actual
net asset value of their portfolio securities and their shares may have greater
volatility because of the potential lack of liquidity.

Exchange Traded Fund Risk: The risks of owning an ETF generally reflect the risks
of owning the underlying securities they are designed to track, although lack of
liquidity in an ETF could result in it being more volatile than the underlying
portfolio of securities. Disruptions in the markets for the securities underlying
ETFs purchased or sold by the Fund could result in losses on the Fund's investment
in ETFs. ETFs also have management fees that increase their costs versus the costs
of owning the underlying securities directly.

Zero Coupon Bond Risk: Because zero-coupon securities bear no interest and
compound semi-annually at the rate fixed at the time of issuance, their value
generally is more volatile than the value of other fixed income securities. An
investment in zero-coupon and delayed interest securities may cause the Fund to
recognize income, and therefore the Fund may be required to make distributions
to shareholders before the Fund receives any cash payments on its investment.

Mortgage-Backed/Asset-Backed Securities Risk: Prepayment risk is associated
with mortgage-backed and asset-backed securities. If interest rates fall, the
underlying debt may be repaid ahead of schedule, reducing the value of the
Fund's investments. If interest rates rise, there may be fewer prepayments,
which would cause the average bond maturity to rise, increasing the potential
for the Fund to lose money. The value of these securities may be significantly
affected by changes in interest rates, the market's perception of issuers, and
the creditworthiness of the parties involved. The ability of the Fund to
successfully utilize these instruments may depend on the ability of the
Subadvisor to forecast interest rates and other economic factors correctly.
These securities may have a structure that makes their reaction to interest
rate changes and other factors difficult to predict, making their value highly
volatile.

Municipal Bond Risk: Municipal bond risks include the ability of the issuer to
repay the obligation, the relative lack of information about certain issuers,
and the possibility of future tax and legislative changes, which could affect
the market for and value of municipal securities. Municipalities continue to
experience economic and financial difficulties in the current economic
environment. The ability of a municipal issuer to make payments and the value
of municipal bonds can be affected by uncertainties in the municipal securities
market. Such uncertainties could cause increased volatility in the municipal
securities market and could negatively impact the Fund's net asset value.

Derivatives Risk: Derivatives are investments whose value depends on (or is
derived from) the value of an underlying instrument, such as a security, asset,
reference rate or index. Derivative strategies often involve leverage, which
may exaggerate a loss, potentially causing the Fund to lose more money than it
would have lost had it invested in the underlying instrument. Derivatives may
be difficult to sell, unwind or value. Derivatives may also be subject to
counterparty risk, which is the risk that the counterparty (the party on the
other side of the transaction) on a derivative transaction will be unable to
honor its contractual obligations to the Fund. Swap transactions tend to shift
the Fund's investment exposure from one type of investment to another, and
therefore entail the risk that a party will default on its payment obligations
to the Fund.

Futures may be more volatile than direct investments in the instrument
underlying the futures, and may not correlate perfectly to the underlying
instrument. Futures also may involve a small initial investment relative to the
risk assumed, which could result in losses greater than if they had not been
used. Due to fluctuations in the price of the underlying security, the Fund may
not be able to profitably exercise an option and may lose its entire investment
in an option. Forward commitments entail the risk that the instrument may be
worth less when it is issued or received than the price the Fund agreed to pay
when it made the commitment. The use of foreign currency forwards may result in
currency exchange losses due to fluctuations in currency exchange rates or an
imperfect correlation between portfolio holdings denominated in a particular
currency and the forward contracts entered into by the Fund.

Tax Risk: The Fund's investments and investment strategies, including
transactions in options and futures contracts, may be subject to special and
complex federal income tax provisions, the effect of which may be, among other
things: (1) to disallow, suspend, defer or otherwise limit the allowance of
certain losses or deductions; (2) to accelerate income to the Fund; (3) to
convert long-term capital gain, which is currently subject to lower tax rates,
into short-term capital gain or ordinary income, which are currently subject to
higher tax rates; (4) to convert an ordinary loss or a deduction into a capital
loss (the deductibility of which is more limited); (5) to treat dividends that
would otherwise constitute qualified dividend income as non-qualified dividend
income; (6) to produce income that will not qualify as good income under the
gross income requirements that must be met for the Fund to qualify as a
regulated investment company (a "RIC") under Subchapter M of the Internal
Revenue Code of 1986, as amended (the "Code"). Furthermore, to the extent that
any futures contract or option on a futures contract held by the Fund is a
"section 1256 contract" under Section 1256 of the Code, the contract will be
marked to market annually, and any gain or loss will be treated as 60% long-term
and 40% short-term, regardless of the holding period for such contract. Section
1256 contracts include Fund transactions involving call options on a broad-based
securities index, certain futures contracts and other financial contracts.

Past Performance

The following bar chart and tables indicate some of the risks of investing in
the Fund. The bar chart shows you how the Fund's calendar year performance has
varied over the life of the Fund. Sales loads are not reflected in the bar
chart. If they were, returns would be less than those shown. The average annual
total returns table shows how the Fund's average annual total returns (before
and after taxes) for the one-year period and life of the Fund compare to those
of a broad-based securities market index. The Fund has selected the S&P 500®Index
as its primary benchmark. The S&P 500® Index is widely regarded as the standard
index for measuring large-cap U.S. stock market performance.

The performance figures for Class I shares reflect the historical performance of
the then-existing shares of Marketfield Fund (the predecessor to the Fund, which
was subject to a different fee structure, and for which a predecessor entity to
Marketfield Asset Management LLC served as investment advisor) for periods prior
to October 5, 2012. Performance figures for all other share classes reflect the
historical performance of Class I shares and are adjusted to reflect differences
in fees and expenses. Performance data for the classes varies based on differences
in their fee and expense structures. The Fund commenced operations on July 31, 2007.
Unadjusted, the performance for the newer classes might have been different. Past
performance of Marketfield Fund (before and after taxes) is not necessarily an
indication of how the Fund will perform in the future. Please visit
mainstayinvestments.com for more recent performance information.

The bar chart shows you how the Fund's calendar year performance has varied
over the life of the Fund. Sales loads are not reflected in the bar chart.
If they were, returns would be less than those shown.

Annual Returns, Class I Shares (by calendar year 2008-2011)

Best Quarter    
2Q/09    15.89%
Worst Quarter   
4Q/08   -13.05%

As of June 30, 2012, the Class I shares of the Fund had a year-to-date return of
8.77%.

Average Annual Total Returns (for the periods ended December 31, 2011)

Average Annual Total Returns MainStay Marketfield Fund	Label	1 Year	Since Inception
Investor Class	Return Before Taxes Investor Class	(2.25%)	6.36%
Class A	Return Before Taxes Class A	(2.25%)	6.36%
Class C	Return Before Taxes Class C	1.67%	6.93%
Class I	Return Before Taxes Class I	3.70%	8.00%
Class I After Taxes on Distributions	Return After Taxes on Distributions Class I	3.70%	7.98%
Class I After Taxes on Distributions and Sales	Return After Taxes on Distributions and Sale of Fund Shares Class I	2.40%	6.92%
Class R2	Return Before Taxes Class R2	3.34%	7.62%
S&P 500®Index	S&P 500® Index (reflects no deductions for fees, expenses, or taxes)	2.11%	(1.09%)

After-tax returns are calculated using the historical highest individual federal
marginal income tax rates and do not reflect the impact of state and local taxes.
In some cases, the return after taxes may exceed the return before taxes due to
an assumed tax benefit from any losses on a sale of shares at the end of the
measurement period. Actual after-tax returns depend on your tax situation and
may differ from those shown. After-tax returns are not relevant if you hold
your shares through tax-deferred arrangements, such as 401(k) plans or individual
retirement accounts. After-tax returns shown are for Class I shares. After-tax
returns for the other share classes may vary.

[1] A contingent deferred sales charge of 1.00% may be imposed on certain redemptions made within one year of the date of purchase on shares that were purchased without an initial sales charge.

[1] Effective October 5, 2012, New York Life Investment Management LLC ("New York Life Investments") has contractually agreed to waive fees and/or reimburse expenses so that Total Annual Fund Operating Expenses (excluding taxes, interest, litigation, extraordinary expenses, brokerage and other transaction expenses relating to the purchase or sale of portfolio investments, and acquired (underlying) fund fees and expenses) for Class I shares do not exceed 1.56% of its average daily net assets. This agreement will be in effect for a two-year period unless extended by New York Life Investments and approved by the Board of Trustees.

XML

Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 31, 2012
Registrant Name	dei_EntityRegistrantName	MAINSTAY FUNDS TRUST
Central Index Key	dei_EntityCentralIndexKey	0001469192
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Sep. 27, 2012
Document Effective Date	dei_DocumentEffectiveDate	Sep. 27, 2012
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Investor Class
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MFNDX
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class A
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MFADX
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class C
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MFCDX
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class I
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MFLDX
MainStay Marketfield Fund (Prospectus Summary) | MainStay Marketfield Fund | Class R2
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MFRDX