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Shareholder Report
 6 Months Ended
Jun. 30, 2026
USD ($)
Holding
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name MFS VARIABLE INSURANCE TRUST III
Entity Central Index Key 0001065698
Entity Investment Company Type N-1A
Document Period End Date Jun. 30, 2026
C000021902 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Global Real Estate Portfolio
Class Name Service Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Global Real Estate Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Service Class $60 1.15%
Expenses Paid, Amount $ 60
Expense Ratio, Percent 1.15%
Net Assets $ 155,818,263
Holdings Count | Holding 56
Investment Company Portfolio Turnover 27.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 155,818,263 Portfolio Turnover Rate (%): 27
Total Number of Holdings: 56
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Equities 98.0%
Money Market Funds 1.5%
Purchased Options 0.5%
Top ten holdings
Prologis, Inc., REIT 7.7%
Equinix, Inc., REIT 5.3%
Simon Property Group, Inc., REIT 4.1%
Ventas, Inc., REIT 3.9%
Goodman Group, REIT 3.7%
Digital Realty Trust, Inc., REIT 3.3%
Mitsubishi Estate Co. Ltd. 2.9%
BXP, Inc., REIT 2.9%
Rexford Industrial Realty, Inc., REIT 2.3%
Janus Living, Inc., "A", REIT 2.3%
Issuer country weightings
United States 67.5%
Australia 8.4%
Japan 7.9%
United Kingdom 6.9%
France 3.4%
Singapore 2.0%
Hong Kong 2.0%
Belgium 1.9%
Largest Holdings [Text Block]
Top ten holdings
Prologis, Inc., REIT 7.7%
Equinix, Inc., REIT 5.3%
Simon Property Group, Inc., REIT 4.1%
Ventas, Inc., REIT 3.9%
Goodman Group, REIT 3.7%
Digital Realty Trust, Inc., REIT 3.3%
Mitsubishi Estate Co. Ltd. 2.9%
BXP, Inc., REIT 2.9%
Rexford Industrial Realty, Inc., REIT 2.3%
Janus Living, Inc., "A", REIT 2.3%
C000021901 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Global Real Estate Portfolio
Class Name Initial Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Global Real Estate Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Initial Class $47 0.90%
Expenses Paid, Amount $ 47
Expense Ratio, Percent 0.90%
Net Assets $ 155,818,263
Holdings Count | Holding 56
Investment Company Portfolio Turnover 27.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 155,818,263 Portfolio Turnover Rate (%): 27
Total Number of Holdings: 56
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Equities 98.0%
Money Market Funds 1.5%
Purchased Options 0.5%
Top ten holdings
Prologis, Inc., REIT 7.7%
Equinix, Inc., REIT 5.3%
Simon Property Group, Inc., REIT 4.1%
Ventas, Inc., REIT 3.9%
Goodman Group, REIT 3.7%
Digital Realty Trust, Inc., REIT 3.3%
Mitsubishi Estate Co. Ltd. 2.9%
BXP, Inc., REIT 2.9%
Rexford Industrial Realty, Inc., REIT 2.3%
Janus Living, Inc., "A", REIT 2.3%
Issuer country weightings
United States 67.5%
Australia 8.4%
Japan 7.9%
United Kingdom 6.9%
France 3.4%
Singapore 2.0%
Hong Kong 2.0%
Belgium 1.9%
Largest Holdings [Text Block]
Top ten holdings
Prologis, Inc., REIT 7.7%
Equinix, Inc., REIT 5.3%
Simon Property Group, Inc., REIT 4.1%
Ventas, Inc., REIT 3.9%
Goodman Group, REIT 3.7%
Digital Realty Trust, Inc., REIT 3.3%
Mitsubishi Estate Co. Ltd. 2.9%
BXP, Inc., REIT 2.9%
Rexford Industrial Realty, Inc., REIT 2.3%
Janus Living, Inc., "A", REIT 2.3%
C000021905 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Blended Research®Small Cap Equity Portfolio
Class Name Initial Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Blended Research Small Cap Equity Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Initial Class $31 0.57%
Expenses Paid, Amount $ 31
Expense Ratio, Percent 0.57%
Net Assets $ 86,065,068
Holdings Count | Holding 252
Investment Company Portfolio Turnover 37.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 86,065,068 Portfolio Turnover Rate (%): 37
Total Number of Holdings: 252
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Equities 98.5%
Money Market Funds 1.5%
Equity sectors
Health Care 18.3%
Industrials 17.6%
Information Technology 17.5%
Financials 17.0%
Consumer Discretionary 8.1%
Energy 4.8%
Real Estate 4.8%
Materials 4.8%
Utilities 2.6%
Communication Services 1.6%
Consumer Staples 1.4%
Top ten holdings
Formfactor, Inc. 2.1%
MYR Group, Inc. 1.8%
Advanced Energy Industries, Inc. 1.6%
UMB Financial Corp. 1.6%
Corsair Gaming, Inc. 1.5%
Popular, Inc. 1.5%
Portland General Electric Co. 1.4%
Bloom Energy Corp. 1.3%
Modine Manufacturing Co. 1.4%
Envista Holdings Corp. 1.4%
MFS uses the Global Industry Classification Standard (GICS®) for sector/industry reporting. MFS has applied its own internal sector/industry classification methodology for equity securities and non-equity securities that are unclassified by GICS.
Largest Holdings [Text Block]
Top ten holdings
Formfactor, Inc. 2.1%
MYR Group, Inc. 1.8%
Advanced Energy Industries, Inc. 1.6%
UMB Financial Corp. 1.6%
Corsair Gaming, Inc. 1.5%
Popular, Inc. 1.5%
Portland General Electric Co. 1.4%
Bloom Energy Corp. 1.3%
Modine Manufacturing Co. 1.4%
Envista Holdings Corp. 1.4%
C000021906 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Blended Research®Small Cap Equity Portfolio
Class Name Service Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Blended Research Small Cap Equity Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Service Class $45 0.82%
Expenses Paid, Amount $ 45
Expense Ratio, Percent 0.82%
Net Assets $ 86,065,068
Holdings Count | Holding 252
Investment Company Portfolio Turnover 37.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 86,065,068 Portfolio Turnover Rate (%): 37
Total Number of Holdings: 252
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Equities 98.5%
Money Market Funds 1.5%
Equity sectors
Health Care 18.3%
Industrials 17.6%
Information Technology 17.5%
Financials 17.0%
Consumer Discretionary 8.1%
Energy 4.8%
Real Estate 4.8%
Materials 4.8%
Utilities 2.6%
Communication Services 1.6%
Consumer Staples 1.4%
Top ten holdings
Formfactor, Inc. 2.1%
MYR Group, Inc. 1.8%
Advanced Energy Industries, Inc. 1.6%
UMB Financial Corp. 1.6%
Corsair Gaming, Inc. 1.5%
Popular, Inc. 1.5%
Portland General Electric Co. 1.4%
Bloom Energy Corp. 1.3%
Modine Manufacturing Co. 1.4%
Envista Holdings Corp. 1.4%
MFS uses the Global Industry Classification Standard (GICS®) for sector/industry reporting. MFS has applied its own internal sector/industry classification methodology for equity securities and non-equity securities that are unclassified by GICS.
Largest Holdings [Text Block]
Top ten holdings
Formfactor, Inc. 2.1%
MYR Group, Inc. 1.8%
Advanced Energy Industries, Inc. 1.6%
UMB Financial Corp. 1.6%
Corsair Gaming, Inc. 1.5%
Popular, Inc. 1.5%
Portland General Electric Co. 1.4%
Bloom Energy Corp. 1.3%
Modine Manufacturing Co. 1.4%
Envista Holdings Corp. 1.4%
C000052150 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Mid Cap Value Portfolio
Class Name Service Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Mid Cap Value Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Service Class $54 1.03%
Expenses Paid, Amount $ 54
Expense Ratio, Percent 1.03%
Net Assets $ 657,326,392
Holdings Count | Holding 157
Investment Company Portfolio Turnover 18.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 657,326,392 Portfolio Turnover Rate (%): 18
Total Number of Holdings: 157
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Equities 95.2%
Money Market Funds 4.8%
Equity sectors
Industrials 17.6%
Financials 16.8%
Consumer Discretionary 9.2%
Health Care 9.1%
Information Technology 8.9%
Utilities 7.6%
Real Estate 7.4%
Energy 7.2%
Consumer Staples 5.2%
Materials 4.9%
Communication Services 1.3%
Top ten holdings
Humana, Inc. 1.4%
Northern Trust Corp. 1.2%
Becton, Dickinson and Co. 1.2%
Targa Resources Corp. 1.1%
Valero Energy Corp. 1.1%
Hartford Insurance Group, Inc. 1.1%
PG&E Corp. 1.1%
Agilent Technologies, Inc. 1.1%
Pulte Homes, Inc. 1.1%
M&T Bank Corp. 1.0%
MFS uses the Global Industry Classification Standard (GICS®) for sector/industry reporting. MFS has applied its own internal sector/industry classification methodology for equity securities and non-equity securities that are unclassified by GICS.
Largest Holdings [Text Block]
Top ten holdings
Humana, Inc. 1.4%
Northern Trust Corp. 1.2%
Becton, Dickinson and Co. 1.2%
Targa Resources Corp. 1.1%
Valero Energy Corp. 1.1%
Hartford Insurance Group, Inc. 1.1%
PG&E Corp. 1.1%
Agilent Technologies, Inc. 1.1%
Pulte Homes, Inc. 1.1%
M&T Bank Corp. 1.0%
C000052149 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Mid Cap Value Portfolio
Class Name Initial Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Mid Cap Value Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Initial Class $41 0.78%
Expenses Paid, Amount $ 41
Expense Ratio, Percent 0.78%
Net Assets $ 657,326,392
Holdings Count | Holding 157
Investment Company Portfolio Turnover 18.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 657,326,392 Portfolio Turnover Rate (%): 18
Total Number of Holdings: 157
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Equities 95.2%
Money Market Funds 4.8%
Equity sectors
Industrials 17.6%
Financials 16.8%
Consumer Discretionary 9.2%
Health Care 9.1%
Information Technology 8.9%
Utilities 7.6%
Real Estate 7.4%
Energy 7.2%
Consumer Staples 5.2%
Materials 4.9%
Communication Services 1.3%
Top ten holdings
Humana, Inc. 1.4%
Northern Trust Corp. 1.2%
Becton, Dickinson and Co. 1.2%
Targa Resources Corp. 1.1%
Valero Energy Corp. 1.1%
Hartford Insurance Group, Inc. 1.1%
PG&E Corp. 1.1%
Agilent Technologies, Inc. 1.1%
Pulte Homes, Inc. 1.1%
M&T Bank Corp. 1.0%
MFS uses the Global Industry Classification Standard (GICS®) for sector/industry reporting. MFS has applied its own internal sector/industry classification methodology for equity securities and non-equity securities that are unclassified by GICS.
Largest Holdings [Text Block]
Top ten holdings
Humana, Inc. 1.4%
Northern Trust Corp. 1.2%
Becton, Dickinson and Co. 1.2%
Targa Resources Corp. 1.1%
Valero Energy Corp. 1.1%
Hartford Insurance Group, Inc. 1.1%
PG&E Corp. 1.1%
Agilent Technologies, Inc. 1.1%
Pulte Homes, Inc. 1.1%
M&T Bank Corp. 1.0%
C000052151 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Limited Maturity Portfolio
Class Name Initial Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Limited Maturity Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Initial Class $24 0.48%
Expenses Paid, Amount $ 24
Expense Ratio, Percent 0.48%
Net Assets $ 282,297,183
Holdings Count | Holding 335
Investment Company Portfolio Turnover 30.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 282,297,183 Average Effective Maturity (yrs): 2.6
Total Number of Holdings: 335 Average Effective Duration (yrs): 1.9
Portfolio Turnover Rate (%): 30
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Fixed Income 95.6%
Money Market Funds 4.4%
Composition including fixed income credit quality
AAA 11.4%
AA 10.8%
A 18.4%
BBB 33.1%
U.S. Government 21.0%
Federal Agencies 0.9%
Money Market Funds 4.4%
Ratings are assigned using Moody’s, Fitch, and Standard & Poor’s and applying a hierarchy: If all 3 agencies rate a security, the middle rating is assigned; if 2 agencies rate a security, the lower rating is assigned. If none of the 3 agencies rate a security, we use DBRS Morningstar. If none of the 4 agencies rate a security, we use Kroll Bond Rating Agency. Ratings are shown in the S&P and Fitch scale (e.g., AAA). All ratings are subject to change.
Credit Ratings Selection [Text Block]
Ratings are assigned using Moody’s, Fitch, and Standard & Poor’s and applying a hierarchy: If all 3 agencies rate a security, the middle rating is assigned; if 2 agencies rate a security, the lower rating is assigned. If none of the 3 agencies rate a security, we use DBRS Morningstar. If none of the 4 agencies rate a security, we use Kroll Bond Rating Agency. Ratings are shown in the S&P and Fitch scale (e.g., AAA). All ratings are subject to change.
C000052152 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Limited Maturity Portfolio
Class Name Service Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Limited Maturity Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Service Class $36 0.73%
Expenses Paid, Amount $ 36
Expense Ratio, Percent 0.73%
Net Assets $ 282,297,183
Holdings Count | Holding 335
Investment Company Portfolio Turnover 30.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 282,297,183 Average Effective Maturity (yrs): 2.6
Total Number of Holdings: 335 Average Effective Duration (yrs): 1.9
Portfolio Turnover Rate (%): 30
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Fixed Income 95.6%
Money Market Funds 4.4%
Composition including fixed income credit quality
AAA 11.4%
AA 10.8%
A 18.4%
BBB 33.1%
U.S. Government 21.0%
Federal Agencies 0.9%
Money Market Funds 4.4%
Ratings are assigned using Moody’s, Fitch, and Standard & Poor’s and applying a hierarchy: If all 3 agencies rate a security, the middle rating is assigned; if 2 agencies rate a security, the lower rating is assigned. If none of the 3 agencies rate a security, we use DBRS Morningstar. If none of the 4 agencies rate a security, we use Kroll Bond Rating Agency. Ratings are shown in the S&P and Fitch scale (e.g., AAA). All ratings are subject to change.
Credit Ratings Selection [Text Block]
Ratings are assigned using Moody’s, Fitch, and Standard & Poor’s and applying a hierarchy: If all 3 agencies rate a security, the middle rating is assigned; if 2 agencies rate a security, the lower rating is assigned. If none of the 3 agencies rate a security, we use DBRS Morningstar. If none of the 4 agencies rate a security, we use Kroll Bond Rating Agency. Ratings are shown in the S&P and Fitch scale (e.g., AAA). All ratings are subject to change.
C000068441 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® ConservativeAllocation Portfolio
Class Name Service Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Conservative Allocation Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment ~ Costs paid as a percentage of a $10,000 investment (annualized) ~
Service Class $16 0.31%
˜ Expenses are direct expenses and do not include underlying fund fees and expenses.
Expenses Paid, Amount $ 16 [1]
Expense Ratio, Percent 0.31% [1]
Net Assets $ 214,145,297
Holdings Count | Holding 18
Investment Company Portfolio Turnover 1.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 214,145,297 Portfolio Turnover Rate (%): 1
Total Number of Holdings: 18
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio allocation
Port Comp - Portfolio Target Allocation Chart
Portfolio holdings
MFS Total Return Bond Series 15.9%
MFS Limited Maturity Portfolio 11.0%
MFS Inflation-Adjusted Bond Portfolio 10.0%
MFS Government Securities Portfolio 10.0%
MFS Global Governments Portfolio 8.0%
MFS Research Series 6.3%
MFS Value Series 6.0%
MFS Research International Portfolio 5.3%
MFS Growth Series 5.2%
MFS High Yield Portfolio 5.0%
MFS Mid Cap Growth Series 4.1%
MFS Mid Cap Value Portfolio 4.0%
MFS International Growth Portfolio 2.1%
MFS International Intrinsic Equity Portfolio 2.1%
MFS Global Real Estate Portfolio 2.0%
MFS New Discovery Series 1.0%
MFS New Discovery Value Portfolio 1.0%
MFS Institutional Money Market Portfolio 1.0%
C000068440 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® ConservativeAllocation Portfolio
Class Name Initial Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Conservative Allocation Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment ~ Costs paid as a percentage of a $10,000 investment (annualized) ~
Initial Class $3 0.06%
˜ Expenses are direct expenses and do not include underlying fund fees and expenses.
Expenses Paid, Amount $ 3 [2]
Expense Ratio, Percent 0.06% [2]
Net Assets $ 214,145,297
Holdings Count | Holding 18
Investment Company Portfolio Turnover 1.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 214,145,297 Portfolio Turnover Rate (%): 1
Total Number of Holdings: 18
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio allocation
Port Comp - Portfolio Target Allocation Chart
Portfolio holdings
MFS Total Return Bond Series 15.9%
MFS Limited Maturity Portfolio 11.0%
MFS Inflation-Adjusted Bond Portfolio 10.0%
MFS Government Securities Portfolio 10.0%
MFS Global Governments Portfolio 8.0%
MFS Research Series 6.3%
MFS Value Series 6.0%
MFS Research International Portfolio 5.3%
MFS Growth Series 5.2%
MFS High Yield Portfolio 5.0%
MFS Mid Cap Growth Series 4.1%
MFS Mid Cap Value Portfolio 4.0%
MFS International Growth Portfolio 2.1%
MFS International Intrinsic Equity Portfolio 2.1%
MFS Global Real Estate Portfolio 2.0%
MFS New Discovery Series 1.0%
MFS New Discovery Value Portfolio 1.0%
MFS Institutional Money Market Portfolio 1.0%
C000068442 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® ModerateAllocation Portfolio
Class Name Initial Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Moderate Allocation Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment ~ Costs paid as a percentage of a $10,000 investment (annualized) ~
Initial Class $1 0.02%
˜ Expenses are direct expenses and do not include underlying fund fees and expenses.
Expenses Paid, Amount $ 1 [3]
Expense Ratio, Percent 0.02% [3]
Net Assets $ 822,676,728
Holdings Count | Holding 18
Investment Company Portfolio Turnover 1.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 822,676,728 Portfolio Turnover Rate (%): 1
Total Number of Holdings: 18
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio allocation
Port Comp - Portfolio Target Allocation Chart
Portfolio holdings
MFS Total Return Bond Series 11.9%
MFS Government Securities Portfolio 9.9%
MFS Value Series 9.0%
MFS Research International Portfolio 8.3%
MFS Growth Series 8.2%
MFS Research Series 7.3%
MFS Mid Cap Growth Series 7.1%
MFS Mid Cap Value Portfolio 7.1%
MFS High Yield Portfolio 5.0%
MFS Global Governments Portfolio 5.0%
MFS Inflation-Adjusted Bond Portfolio 5.0%
MFS International Growth Portfolio 3.1%
MFS International Intrinsic Equity Portfolio 3.1%
MFS Global Real Estate Portfolio 3.0%
MFS Limited Maturity Portfolio 3.0%
MFS New Discovery Series 1.5%
MFS New Discovery Value Portfolio 1.5%
MFS Institutional Money Market Portfolio 1.0%
C000068443 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® ModerateAllocation Portfolio
Class Name Service Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Moderate Allocation Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment ~ Costs paid as a percentage of a $10,000 investment (annualized) ~
Service Class $14 0.27%
˜ Expenses are direct expenses and do not include underlying fund fees and expenses.
Expenses Paid, Amount $ 14 [4]
Expense Ratio, Percent 0.27% [4]
Net Assets $ 822,676,728
Holdings Count | Holding 18
Investment Company Portfolio Turnover 1.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 822,676,728 Portfolio Turnover Rate (%): 1
Total Number of Holdings: 18
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio allocation
Port Comp - Portfolio Target Allocation Chart
Portfolio holdings
MFS Total Return Bond Series 11.9%
MFS Government Securities Portfolio 9.9%
MFS Value Series 9.0%
MFS Research International Portfolio 8.3%
MFS Growth Series 8.2%
MFS Research Series 7.3%
MFS Mid Cap Growth Series 7.1%
MFS Mid Cap Value Portfolio 7.1%
MFS High Yield Portfolio 5.0%
MFS Global Governments Portfolio 5.0%
MFS Inflation-Adjusted Bond Portfolio 5.0%
MFS International Growth Portfolio 3.1%
MFS International Intrinsic Equity Portfolio 3.1%
MFS Global Real Estate Portfolio 3.0%
MFS Limited Maturity Portfolio 3.0%
MFS New Discovery Series 1.5%
MFS New Discovery Value Portfolio 1.5%
MFS Institutional Money Market Portfolio 1.0%
C000068445 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Growth Allocation Portfolio
Class Name Service Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Growth Allocation Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment ~ Costs paid as a percentage of a $10,000 investment (annualized) ~
Service Class $15 0.29%
˜ Expenses are direct expenses and do not include underlying fund fees and expenses.
Expenses Paid, Amount $ 15 [5]
Expense Ratio, Percent 0.29% [5]
Net Assets $ 369,566,563
Holdings Count | Holding 18
Investment Company Portfolio Turnover 3.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 369,566,563 Portfolio Turnover Rate (%): 3
Total Number of Holdings: 18
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio allocation
Port Comp - Portfolio Target Allocation Chart
Portfolio holdings
MFS Value Series 11.0%
MFS Growth Series 10.3%
MFS Research International Portfolio 10.3%
MFS Research Series 9.2%
MFS Mid Cap Growth Series 9.2%
MFS Mid Cap Value Portfolio 9.1%
MFS International Growth Portfolio 5.1%
MFS International Intrinsic Equity Portfolio 5.0%
MFS Global Real Estate Portfolio 5.0%
MFS Total Return Bond Series 5.0%
MFS High Yield Portfolio 4.9%
MFS Inflation-Adjusted Bond Portfolio 4.9%
MFS Global Governments Portfolio 3.9%
MFS New Discovery Series 2.1%
MFS New Discovery Value Portfolio 2.0%
MFS Emerging Markets Equity Portfolio 1.0%
MFS Institutional Money Market Portfolio 1.0%
MFS Limited Maturity Portfolio 1.0%
C000068444 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Growth Allocation Portfolio
Class Name Initial Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Growth Allocation Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment ~ Costs paid as a percentage of a $10,000 investment (annualized) ~
Initial Class $2 0.04%
˜ Expenses are direct expenses and do not include underlying fund fees and expenses.
Expenses Paid, Amount $ 2 [6]
Expense Ratio, Percent 0.04% [6]
Net Assets $ 369,566,563
Holdings Count | Holding 18
Investment Company Portfolio Turnover 3.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 369,566,563 Portfolio Turnover Rate (%): 3
Total Number of Holdings: 18
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio allocation
Port Comp - Portfolio Target Allocation Chart
Portfolio holdings
MFS Value Series 11.0%
MFS Growth Series 10.3%
MFS Research International Portfolio 10.3%
MFS Research Series 9.2%
MFS Mid Cap Growth Series 9.2%
MFS Mid Cap Value Portfolio 9.1%
MFS International Growth Portfolio 5.1%
MFS International Intrinsic Equity Portfolio 5.0%
MFS Global Real Estate Portfolio 5.0%
MFS Total Return Bond Series 5.0%
MFS High Yield Portfolio 4.9%
MFS Inflation-Adjusted Bond Portfolio 4.9%
MFS Global Governments Portfolio 3.9%
MFS New Discovery Series 2.1%
MFS New Discovery Value Portfolio 2.0%
MFS Emerging Markets Equity Portfolio 1.0%
MFS Institutional Money Market Portfolio 1.0%
MFS Limited Maturity Portfolio 1.0%
C000068446 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® New DiscoveryValue Portfolio
Class Name Initial Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS New Discovery Value Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Initial Class $47 0.88%
Expenses Paid, Amount $ 47
Expense Ratio, Percent 0.88%
Net Assets $ 46,774,690
Holdings Count | Holding 148
Investment Company Portfolio Turnover 29.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 46,774,690 Portfolio Turnover Rate (%): 29
Total Number of Holdings: 148
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Equities 95.0%
Money Market Funds 5.0%
Equity sectors
Financials 23.6%
Industrials 13.0%
Health Care 11.1%
Consumer Discretionary 9.4%
Information Technology 8.4%
Real Estate 7.1%
Energy 6.6%
Utilities 6.1%
Materials 5.6%
Consumer Staples 2.1%
Communication Services 2.0%
Top ten holdings
Element Solutions, Inc. 1.4%
Prosperity Bancshares, Inc. 1.2%
Cathay General Bancorp, Inc. 1.2%
Columbia Banking System, Inc. 1.2%
United Community Bank, Inc. 1.2%
First Interstate BancSystem, Inc. 1.2%
First Hawaiian, Inc. 1.2%
Plains GP Holdings LP 1.2%
M/I Homes, Inc. 1.2%
Hancock Whitney Corp. 1.2%
MFS uses the Global Industry Classification Standard (GICS®) for sector/industry reporting. MFS has applied its own internal sector/industry classification methodology for equity securities and non-equity securities that are unclassified by GICS.
Largest Holdings [Text Block]
Top ten holdings
Element Solutions, Inc. 1.4%
Prosperity Bancshares, Inc. 1.2%
Cathay General Bancorp, Inc. 1.2%
Columbia Banking System, Inc. 1.2%
United Community Bank, Inc. 1.2%
First Interstate BancSystem, Inc. 1.2%
First Hawaiian, Inc. 1.2%
Plains GP Holdings LP 1.2%
M/I Homes, Inc. 1.2%
Hancock Whitney Corp. 1.2%
C000068447 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® New DiscoveryValue Portfolio
Class Name Service Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS New Discovery Value Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Service Class $60 1.13%
Expenses Paid, Amount $ 60
Expense Ratio, Percent 1.13%
Net Assets $ 46,774,690
Holdings Count | Holding 148
Investment Company Portfolio Turnover 29.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 46,774,690 Portfolio Turnover Rate (%): 29
Total Number of Holdings: 148
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Equities 95.0%
Money Market Funds 5.0%
Equity sectors
Financials 23.6%
Industrials 13.0%
Health Care 11.1%
Consumer Discretionary 9.4%
Information Technology 8.4%
Real Estate 7.1%
Energy 6.6%
Utilities 6.1%
Materials 5.6%
Consumer Staples 2.1%
Communication Services 2.0%
Top ten holdings
Element Solutions, Inc. 1.4%
Prosperity Bancshares, Inc. 1.2%
Cathay General Bancorp, Inc. 1.2%
Columbia Banking System, Inc. 1.2%
United Community Bank, Inc. 1.2%
First Interstate BancSystem, Inc. 1.2%
First Hawaiian, Inc. 1.2%
Plains GP Holdings LP 1.2%
M/I Homes, Inc. 1.2%
Hancock Whitney Corp. 1.2%
MFS uses the Global Industry Classification Standard (GICS®) for sector/industry reporting. MFS has applied its own internal sector/industry classification methodology for equity securities and non-equity securities that are unclassified by GICS.
Largest Holdings [Text Block]
Top ten holdings
Element Solutions, Inc. 1.4%
Prosperity Bancshares, Inc. 1.2%
Cathay General Bancorp, Inc. 1.2%
Columbia Banking System, Inc. 1.2%
United Community Bank, Inc. 1.2%
First Interstate BancSystem, Inc. 1.2%
First Hawaiian, Inc. 1.2%
Plains GP Holdings LP 1.2%
M/I Homes, Inc. 1.2%
Hancock Whitney Corp. 1.2%
C000068455 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Inflation-AdjustedBond Portfolio
Class Name Service Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Inflation-Adjusted Bond Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Service Class $43 0.86%
Expenses Paid, Amount $ 43
Expense Ratio, Percent 0.86%
Net Assets $ 148,187,823
Holdings Count | Holding 102
Investment Company Portfolio Turnover 14.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 148,187,823 Average Effective Maturity (yrs): 9.5
Total Number of Holdings: 102 Average Effective Duration (yrs): 8.5
Portfolio Turnover Rate (%): 14
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Fixed Income 99.9%
Money Market Funds 0.1%
Issuer country weightings
United States 51.8%
United Kingdom 20.5%
Italy 11.2%
Spain 3.9%
France 3.7%
Canada 3.1%
New Zealand 1.4%
Australia 1.0%
Sweden 0.9%
Other Countries 2.5%
Composition including fixed income credit quality
AAA 7.7%
AA 22.0%
A 8.1%
BBB 12.1%
BB 0.6%
U.S. Government 46.4%
Federal Agencies 2.3%
Not Rated 0.7%
Money Market Funds 0.1%
Ratings are assigned using Moody’s, Fitch, and Standard & Poor’s and applying a hierarchy: If all 3 agencies rate a security, the middle rating is assigned; if 2 agencies rate a security, the lower rating is assigned. If none of the 3 agencies rate a security, we use DBRS Morningstar. If none of the 4 agencies rate a security, we use Kroll Bond Rating Agency. Ratings are shown in the S&P and Fitch scale (e.g., AAA). All ratings are subject to change.
Credit Ratings Selection [Text Block]
Ratings are assigned using Moody’s, Fitch, and Standard & Poor’s and applying a hierarchy: If all 3 agencies rate a security, the middle rating is assigned; if 2 agencies rate a security, the lower rating is assigned. If none of the 3 agencies rate a security, we use DBRS Morningstar. If none of the 4 agencies rate a security, we use Kroll Bond Rating Agency. Ratings are shown in the S&P and Fitch scale (e.g., AAA). All ratings are subject to change.
C000068454 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Inflation-AdjustedBond Portfolio
Class Name Initial Class
No Trading Symbol Flag true
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Inflation-Adjusted Bond Portfolio for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at insurancefunds.mfs.com. You can also request this information by contacting us at 1‑800‑225‑2606 or by sending an e-mail request to orderliterature@mfs.com.
Additional Information Phone Number 1‑800‑225‑2606
Additional Information Email orderliterature@mfs.com
Additional Information Website insurancefunds.mfs.com
Expenses [Text Block]
FUND EXPENSES
What were the fund costs for the last six months?
(based on a hypothetical $10,000 investment)
Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment (annualized)
Initial Class $30 0.61%
Expenses Paid, Amount $ 30
Expense Ratio, Percent 0.61%
Net Assets $ 148,187,823
Holdings Count | Holding 102
Investment Company Portfolio Turnover 14.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 6/30/26
Net Assets ($): 148,187,823 Average Effective Maturity (yrs): 9.5
Total Number of Holdings: 102 Average Effective Duration (yrs): 8.5
Portfolio Turnover Rate (%): 14
Where the fund holds derivatives, they are not included in the total number of portfolio holdings.
Holdings [Text Block]
PORTFOLIO COMPOSITION (BASED ON TOTAL INVESTMENTS AS OF 6/30/26)
Portfolio structure
Fixed Income 99.9%
Money Market Funds 0.1%
Issuer country weightings
United States 51.8%
United Kingdom 20.5%
Italy 11.2%
Spain 3.9%
France 3.7%
Canada 3.1%
New Zealand 1.4%
Australia 1.0%
Sweden 0.9%
Other Countries 2.5%
Composition including fixed income credit quality
AAA 7.7%
AA 22.0%
A 8.1%
BBB 12.1%
BB 0.6%
U.S. Government 46.4%
Federal Agencies 2.3%
Not Rated 0.7%
Money Market Funds 0.1%
Ratings are assigned using Moody’s, Fitch, and Standard & Poor’s and applying a hierarchy: If all 3 agencies rate a security, the middle rating is assigned; if 2 agencies rate a security, the lower rating is assigned. If none of the 3 agencies rate a security, we use DBRS Morningstar. If none of the 4 agencies rate a security, we use Kroll Bond Rating Agency. Ratings are shown in the S&P and Fitch scale (e.g., AAA). All ratings are subject to change.
Credit Ratings Selection [Text Block]
Ratings are assigned using Moody’s, Fitch, and Standard & Poor’s and applying a hierarchy: If all 3 agencies rate a security, the middle rating is assigned; if 2 agencies rate a security, the lower rating is assigned. If none of the 3 agencies rate a security, we use DBRS Morningstar. If none of the 4 agencies rate a security, we use Kroll Bond Rating Agency. Ratings are shown in the S&P and Fitch scale (e.g., AAA). All ratings are subject to change.
[1] Expenses are direct expenses and do not include underlying fund fees and expenses.
[2] Expenses are direct expenses and do not include underlying fund fees and expenses.
[3] Expenses are direct expenses and do not include underlying fund fees and expenses.
[4] Expenses are direct expenses and do not include underlying fund fees and expenses.
[5] Expenses are direct expenses and do not include underlying fund fees and expenses.
[6] Expenses are direct expenses and do not include underlying fund fees and expenses.