XML 29 R145.htm IDEA: XBRL DOCUMENT v2.4.0.6
Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading MFS Strategic Income Series Summary of Key Information
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund’s investment objective is to seek total return with an emphasis on high current income, but also considering capital appreciation.

Expense [Heading] rr_ExpenseHeading Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay when you hold shares of the fund. Expenses have been adjusted to reflect the estimated expenses of investing in another MFS fund and to reflect current fee arrangements. If the fees and expenses imposed by the insurance company that issued your variable contracts through which the fund is offered were included, your expenses would be higher.

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):
Management Fee rr_ManagementFeesOverAssets 0.70%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.57%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.53%
Fee Reductions and /or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.47%) [1]
Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements rr_NetExpensesOverAssets 1.06%
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination 2014-04-30
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 rate may indicate higher transaction costs. These transaction 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 40% of the average value of its portfolio.

Portfolio Turnover, Rate rr_PortfolioTurnoverRate 40.00%
Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates Expenses have been adjusted to reflect the estimated expenses of investing in another MFS fund and to reflect current fee arrangements.
Expenses Restated to Reflect Current [Text] rr_ExpensesRestatedToReflectCurrent Expenses have been adjusted to reflect the estimated expenses of investing in another MFS fund and to reflect current fee arrangements.
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. If the fees and expenses imposed by the investment vehicle through which an investment in the fund is made were included, your expenses would be higher.


The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods; your investment has a 5% return each year; and the fund’s operating expenses remain the same.

Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Although your actual costs will likely be higher or lower, under these assumptions your costs would be:
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 108
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 437
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 790
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,784
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests the fund’s assets primarily in debt instruments. MFS may invest the fund’s assets in other mutual funds advised by MFS that invest in securities instead of investing directly in such securities.


MFS normally invests the fund’s assets in U.S. Government securities, foreign government securities, mortgage-backed and other asset-backed securities of U.S. and foreign issuers, corporate bonds of U.S. and/or foreign issuers, and/or debt instruments of issuers located in emerging market countries. MFS allocates the fund’s assets across these categories with a view toward broad diversification across and within these categories. MFS may also invest the fund’s assets in equity securities. Equity securities include common stocks, preferred stocks, securities convertible into stocks, equity interests in real estate investment trusts (REITs), and depositary receipts for such securities.


MFS may invest up to 100% of the fund’s assets directly or indirectly in less than investment grade quality debt instruments (lower quality debt instruments).


MFS may invest a large percentage of the fund’s assets in issuers in a single country, a small number of countries, or a particular geographic region.


While MFS may use derivatives for any investment purpose, to the extent MFS uses derivatives, MFS expects to use derivatives primarily to increase or decrease exposure to a particular market, segment of the market, or security, to increase or decrease interest rate or currency exposure, or as alternatives to direct investments. Derivatives include futures, forward contracts, options, structured securities, inverse floating rate instruments, and swaps.


MFS uses a bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual issuers and instruments. Quantitative models that systematically evaluate issuers and instruments may also be considered.

Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.


The principal risks of investing in the fund are:


Interest Rate Risk:  The price of a debt instrument falls when interest rates rise and rises when interest rates fall. Instruments with longer maturities, or that do not pay current interest, are more sensitive to interest rate changes.


Credit Risk:  The price of a debt instrument depends, in part, on the credit quality of the issuer, borrower, counterparty, or underlying collateral or assets and the terms of the instrument. The price of a debt instrument can decline in response to changes in the financial condition of the issuer, borrower, counterparty, or underlying collateral or assets, or changes in specific or general market, economic, industry, political, regulatory, geopolitical, and other conditions.


Lower quality debt instruments (commonly referred to as “high yield securities” or “junk bonds”) can involve a substantially greater risk of default or can already be in default, and their values can decline significantly. Lower quality debt instruments are regarded as having predominantly speculative characteristics. Lower quality debt instruments tend to be more sensitive to adverse news about the issuer, or the market or economy in general, than higher quality debt instruments.


Foreign and Emerging Markets Risk:  Exposure to foreign markets, especially emerging markets, through issuers or currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, and other conditions. These factors can make foreign investments, especially those in emerging markets, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to these conditions than the U.S. market. Emerging markets can have less developed markets, greater custody and operational risk, less developed legal, regulatory, and accounting systems, and greater political, social, and economic instability than developed markets.


Currency Risk:  The value of foreign currencies relative to the U.S. dollar fluctuates in response to market, economic, industry, political, regulatory, geopolitical, and other conditions, and a decline in the value of a foreign currency versus the U.S. dollar reduces the value in U.S. dollars of investments denominated in that foreign currency.


Geographic Concentration Risk: The fund’s performance could be closely tied to the market, currency, economic, political, regulatory, geopolitical, and other conditions in the countries or regions in which the fund invests and could be more volatile than the performance of more geographically-diversified funds.


Prepayment/Extension Risk:  Instruments subject to prepayment and/or extension can reduce the potential for gain for the instrument’s holders if the instrument is prepaid and increase the potential for loss if the maturity of the instrument is extended.


Inflation-Adjusted Debt Instruments Risk:  Interest payments on inflation-adjusted debt instruments can be unpredictable and vary based on the level of inflation. If inflation is negative, principal and income can both decline.


Municipal Risk:  The price of a municipal instrument can be volatile and significantly affected by adverse tax or court rulings, legislative or political changes, changes in specific or general market and economic conditions, and the financial condition of municipal issuers and insurers. Because many municipal instruments are issued to finance similar projects, conditions in these industries can significantly affect the fund and the overall municipal market.


Stock Market/Company Risk:  Stock markets are volatile and can decline significantly in response to issuer, market, economic, industry, political, regulatory, geopolitical, and other conditions, as well as to investor perceptions of these conditions. The price of an equity security can decrease significantly in response to these conditions, and these conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the market in general.


Derivatives Risk:  Derivatives can be highly volatile and involve risks in addition to the risks of the underlying indicator(s) on which the derivative is based. Gains or losses from derivatives can be substantially greater than the derivatives’ original cost.  Derivatives can involve leverage.


Leveraging Risk:  Leverage involves investment exposure in an amount exceeding the initial investment. Leverage can cause increased volatility by magnifying gains or losses.


Investment Selection Risk: MFS’ investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests.


Counterparty and Third Party Risk:  Transactions involving a counterparty or third party other than the issuer of the instrument are subject to the credit risk of the counterparty or third party, and to the counterparty’s or third party’s ability to perform in accordance with the terms of the transaction.


Liquidity Risk:  It may not be possible to sell certain investments, types of investments, and/or segments of the market at any particular time or at an acceptable price.

Risk Lose Money [Text] rr_RiskLoseMoney As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance Information
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of a broad measure of market performance and one or more other measures of performance for markets in which the fund may invest.


The fund’s past performance does not necessarily indicate how the fund will perform in the future. Updated performance is available at mfs.com or by calling 1-877-411-3325. If the fees and expenses imposed by the investment vehicle through which an investment in the fund is made were included, they would reduce the returns shown.

Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund's performance over time and how the fund's performance over time compares with that of a broad measure of market performance and one or more other measures of performance for markets in which the fund may invest.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-877-411-3325
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress mfs.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The fund's past performance does not necessarily indicate how the fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Service Class Bar Chart.
Annual Return 2003 rr_AnnualReturn2003 10.10%
Annual Return 2004 rr_AnnualReturn2004 7.54%
Annual Return 2005 rr_AnnualReturn2005 1.62%
Annual Return 2006 rr_AnnualReturn2006 6.39%
Annual Return 2007 rr_AnnualReturn2007 3.42%
Annual Return 2008 rr_AnnualReturn2008 (12.26%)
Annual Return 2009 rr_AnnualReturn2009 23.88%
Annual Return 2010 rr_AnnualReturn2010 9.79%
Annual Return 2011 rr_AnnualReturn2011 4.53%
Annual Return 2012 rr_AnnualReturn2012 10.60%
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

The total return for the three-month period ended March 31, 2013 was 0.39%. During the period(s) shown in the bar chart, the highest quarterly return was 9.87% (for the calendar quarter ended June 30, 2009) and the lowest quarterly return was (7.13)% (for the calendar quarter ended December 31, 2008).

Year to Date Return, Label rr_YearToDateReturnLabel The total return for the three-month period ended
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Mar. 31, 2013
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 0.39%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest quarterly return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 9.87%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (7.13%)
Performance Table Heading rr_PerformanceTableHeading Performance Table.
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes (Reflects no deduction for fees, expenses or taxes)
Caption rr_AverageAnnualReturnCaption Average Annual Total Returns (for the Periods Ended December 31, 2012)
Label rr_AverageAnnualReturnLabel Service Class Shares
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 10.60%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 6.65%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 6.21%
Barclays U.S. High-Yield Corporate Bond 2% Issuer Capped Index
  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Index Comparisons (Reflects no deduction for fees, expenses or taxes) Barclays U.S. High-Yield Corporate Bond 2% Issuer Capped Index
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 15.78%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 10.45%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 10.60%
MFS Strategic Income Blended Index
  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Index Comparisons (Reflects no deduction for fees, expenses or taxes) MFS Strategic Income Blended Index
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 9.76% [2]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 8.03% [2]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 7.98% [2]
[1] Massachusetts Financial Services Company has agreed in writing to bear the fund's expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs, and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund's investment activity, and fees and expenses associated with investments in investment companies and other similar investment vehicles), such that "Total Annual Fund Operating Expenses" do not exceed 1.05% of the fund's average daily net assets annually for Service Class shares. This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least April 30, 2014.
[2] As of December 31, 2012, the MFS Strategic Income Blended Index (the "Blended Index") consisted of the following indices and weightings: 10% Barclays U.S. Credit Bond Index, 14% JPMorgan Emerging Market Bond Index Global, 8.5% Citigroup World Government Bond Non-Dollar Index, 8.5% Citigroup World Government Bond Non-Dollar Hedged Index, 26% Barclays U.S. Government/Mortgage Bond Index and 33% Barclays U.S. High-Yield Corporate Bond 2% Issuer Capped Index. The components and weightings of the Blended Index may have differed during the period, and may differ in the future.