v3.26.1
Shareholder Report
 6 Months Ended
Jan. 31, 2026
USD ($)
Holding
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name MFS SERIES TRUST X
Entity Central Index Key 0000783740
Entity Investment Company Type N-1A
Document Period End Date Jan. 31, 2026
C000034483 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Emerging MarketsDebt Fund
Class Name Class R6
Trading Symbol MEDHX
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Emerging Markets Debt Fund for the period of August 1, 2025 to January 31, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at funds.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 funds.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)
R6 $36 0.69%
Expenses Paid, Amount $ 36
Expense Ratio, Percent 0.69%
Net Assets $ 7,581,382,666
Holdings Count | Holding 469
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 1/31/26
Net Assets ($): 7,581,382,666 Average Effective Maturity (yrs): 9.3
Total Number of Holdings: 469 Average Effective Duration (yrs): 5.8
Portfolio Turnover Rate (%): 45
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 1/31/26)
Portfolio structure
Fixed Income 96.0%
Money Market Funds 4.0%
Equities 0.0%
Issuer country weightings
Mexico 5.9%
United States 5.6%
Chile 4.5%
Saudi Arabia 4.3%
Argentina 3.7%
India 3.6%
Romania 3.5%
Colombia 3.5%
Turkey 3.5%
Other Countries 61.9%
Composition including fixed income credit
quality
AA 2.7%
A 11.2%
BBB 29.3%
BB 30.1%
B 12.5%
CCC 7.6%
C 0.7%
U.S. Government 1.5%
Not Rated 0.4%
Non-Fixed Income 0.0%
Money Market Funds 4.0%
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.
C000074227 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Emerging MarketsDebt Fund
Class Name Class R4
Trading Symbol MEDGX
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Emerging Markets Debt Fund for the period of August 1, 2025 to January 31, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at funds.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 funds.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)
R4 $42 0.80%
Expenses Paid, Amount $ 42
Expense Ratio, Percent 0.80%
Net Assets $ 7,581,382,666
Holdings Count | Holding 469
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 1/31/26
Net Assets ($): 7,581,382,666 Average Effective Maturity (yrs): 9.3
Total Number of Holdings: 469 Average Effective Duration (yrs): 5.8
Portfolio Turnover Rate (%): 45
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 1/31/26)
Portfolio structure
Fixed Income 96.0%
Money Market Funds 4.0%
Equities 0.0%
Issuer country weightings
Mexico 5.9%
United States 5.6%
Chile 4.5%
Saudi Arabia 4.3%
Argentina 3.7%
India 3.6%
Romania 3.5%
Colombia 3.5%
Turkey 3.5%
Other Countries 61.9%
Composition including fixed income credit
quality
AA 2.7%
A 11.2%
BBB 29.3%
BB 30.1%
B 12.5%
CCC 7.6%
C 0.7%
U.S. Government 1.5%
Not Rated 0.4%
Non-Fixed Income 0.0%
Money Market Funds 4.0%
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.
C000074226 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Emerging MarketsDebt Fund
Class Name Class R3
Trading Symbol MEDFX
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Emerging Markets Debt Fund for the period of August 1, 2025 to January 31, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at funds.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 funds.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)
R3 $55 1.05%
Expenses Paid, Amount $ 55
Expense Ratio, Percent 1.05%
Net Assets $ 7,581,382,666
Holdings Count | Holding 469
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 1/31/26
Net Assets ($): 7,581,382,666 Average Effective Maturity (yrs): 9.3
Total Number of Holdings: 469 Average Effective Duration (yrs): 5.8
Portfolio Turnover Rate (%): 45
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 1/31/26)
Portfolio structure
Fixed Income 96.0%
Money Market Funds 4.0%
Equities 0.0%
Issuer country weightings
Mexico 5.9%
United States 5.6%
Chile 4.5%
Saudi Arabia 4.3%
Argentina 3.7%
India 3.6%
Romania 3.5%
Colombia 3.5%
Turkey 3.5%
Other Countries 61.9%
Composition including fixed income credit
quality
AA 2.7%
A 11.2%
BBB 29.3%
BB 30.1%
B 12.5%
CCC 7.6%
C 0.7%
U.S. Government 1.5%
Not Rated 0.4%
Non-Fixed Income 0.0%
Money Market Funds 4.0%
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.
C000074225 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Emerging MarketsDebt Fund
Class Name Class R2
Trading Symbol MEDEX
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Emerging Markets Debt Fund for the period of August 1, 2025 to January 31, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at funds.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 funds.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)
R2 $68 1.30%
Expenses Paid, Amount $ 68
Expense Ratio, Percent 1.30%
Net Assets $ 7,581,382,666
Holdings Count | Holding 469
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 1/31/26
Net Assets ($): 7,581,382,666 Average Effective Maturity (yrs): 9.3
Total Number of Holdings: 469 Average Effective Duration (yrs): 5.8
Portfolio Turnover Rate (%): 45
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 1/31/26)
Portfolio structure
Fixed Income 96.0%
Money Market Funds 4.0%
Equities 0.0%
Issuer country weightings
Mexico 5.9%
United States 5.6%
Chile 4.5%
Saudi Arabia 4.3%
Argentina 3.7%
India 3.6%
Romania 3.5%
Colombia 3.5%
Turkey 3.5%
Other Countries 61.9%
Composition including fixed income credit
quality
AA 2.7%
A 11.2%
BBB 29.3%
BB 30.1%
B 12.5%
CCC 7.6%
C 0.7%
U.S. Government 1.5%
Not Rated 0.4%
Non-Fixed Income 0.0%
Money Market Funds 4.0%
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.
C000074224 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Emerging MarketsDebt Fund
Class Name Class R1
Trading Symbol MEDDX
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Emerging Markets Debt Fund for the period of August 1, 2025 to January 31, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at funds.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 funds.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)
R1 $94 1.80%
Expenses Paid, Amount $ 94
Expense Ratio, Percent 1.80%
Net Assets $ 7,581,382,666
Holdings Count | Holding 469
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 1/31/26
Net Assets ($): 7,581,382,666 Average Effective Maturity (yrs): 9.3
Total Number of Holdings: 469 Average Effective Duration (yrs): 5.8
Portfolio Turnover Rate (%): 45
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 1/31/26)
Portfolio structure
Fixed Income 96.0%
Money Market Funds 4.0%
Equities 0.0%
Issuer country weightings
Mexico 5.9%
United States 5.6%
Chile 4.5%
Saudi Arabia 4.3%
Argentina 3.7%
India 3.6%
Romania 3.5%
Colombia 3.5%
Turkey 3.5%
Other Countries 61.9%
Composition including fixed income credit
quality
AA 2.7%
A 11.2%
BBB 29.3%
BB 30.1%
B 12.5%
CCC 7.6%
C 0.7%
U.S. Government 1.5%
Not Rated 0.4%
Non-Fixed Income 0.0%
Money Market Funds 4.0%
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.
C000006903 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Emerging MarketsDebt Fund
Class Name Class I
Trading Symbol MEDIX
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Emerging Markets Debt Fund for the period of August 1, 2025 to January 31, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at funds.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 funds.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)
I $42 0.80%
Expenses Paid, Amount $ 42
Expense Ratio, Percent 0.80%
Net Assets $ 7,581,382,666
Holdings Count | Holding 469
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 1/31/26
Net Assets ($): 7,581,382,666 Average Effective Maturity (yrs): 9.3
Total Number of Holdings: 469 Average Effective Duration (yrs): 5.8
Portfolio Turnover Rate (%): 45
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 1/31/26)
Portfolio structure
Fixed Income 96.0%
Money Market Funds 4.0%
Equities 0.0%
Issuer country weightings
Mexico 5.9%
United States 5.6%
Chile 4.5%
Saudi Arabia 4.3%
Argentina 3.7%
India 3.6%
Romania 3.5%
Colombia 3.5%
Turkey 3.5%
Other Countries 61.9%
Composition including fixed income credit
quality
AA 2.7%
A 11.2%
BBB 29.3%
BB 30.1%
B 12.5%
CCC 7.6%
C 0.7%
U.S. Government 1.5%
Not Rated 0.4%
Non-Fixed Income 0.0%
Money Market Funds 4.0%
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.
C000006902 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Emerging MarketsDebt Fund
Class Name Class C
Trading Symbol MEDCX
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Emerging Markets Debt Fund for the period of August 1, 2025 to January 31, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at funds.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 funds.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)
C $94 1.80%
Expenses Paid, Amount $ 94
Expense Ratio, Percent 1.80%
Net Assets $ 7,581,382,666
Holdings Count | Holding 469
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 1/31/26
Net Assets ($): 7,581,382,666 Average Effective Maturity (yrs): 9.3
Total Number of Holdings: 469 Average Effective Duration (yrs): 5.8
Portfolio Turnover Rate (%): 45
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 1/31/26)
Portfolio structure
Fixed Income 96.0%
Money Market Funds 4.0%
Equities 0.0%
Issuer country weightings
Mexico 5.9%
United States 5.6%
Chile 4.5%
Saudi Arabia 4.3%
Argentina 3.7%
India 3.6%
Romania 3.5%
Colombia 3.5%
Turkey 3.5%
Other Countries 61.9%
Composition including fixed income credit
quality
AA 2.7%
A 11.2%
BBB 29.3%
BB 30.1%
B 12.5%
CCC 7.6%
C 0.7%
U.S. Government 1.5%
Not Rated 0.4%
Non-Fixed Income 0.0%
Money Market Funds 4.0%
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.
C000006901 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Emerging MarketsDebt Fund
Class Name Class B
Trading Symbol MEDBX
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Emerging Markets Debt Fund for the period of August 1, 2025 to January 31, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at funds.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 funds.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)
B $94 1.80%
Expenses Paid, Amount $ 94
Expense Ratio, Percent 1.80%
Net Assets $ 7,581,382,666
Holdings Count | Holding 469
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 1/31/26
Net Assets ($): 7,581,382,666 Average Effective Maturity (yrs): 9.3
Total Number of Holdings: 469 Average Effective Duration (yrs): 5.8
Portfolio Turnover Rate (%): 45
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 1/31/26)
Portfolio structure
Fixed Income 96.0%
Money Market Funds 4.0%
Equities 0.0%
Issuer country weightings
Mexico 5.9%
United States 5.6%
Chile 4.5%
Saudi Arabia 4.3%
Argentina 3.7%
India 3.6%
Romania 3.5%
Colombia 3.5%
Turkey 3.5%
Other Countries 61.9%
Composition including fixed income credit
quality
AA 2.7%
A 11.2%
BBB 29.3%
BB 30.1%
B 12.5%
CCC 7.6%
C 0.7%
U.S. Government 1.5%
Not Rated 0.4%
Non-Fixed Income 0.0%
Money Market Funds 4.0%
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.
C000006900 [Member]  
Shareholder Report [Line Items]  
Fund Name MFS® Emerging MarketsDebt Fund
Class Name Class A
Trading Symbol MEDAX
Annual or Semi-Annual Statement [Text Block] This semiannual shareholder report contains important information about MFS Emerging Markets Debt Fund for the period of August 1, 2025 to January 31, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the fund at funds.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 funds.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)
A $55 1.05%
Expenses Paid, Amount $ 55
Expense Ratio, Percent 1.05%
Net Assets $ 7,581,382,666
Holdings Count | Holding 469
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block]
FUND STATISTICS AS OF 1/31/26
Net Assets ($): 7,581,382,666 Average Effective Maturity (yrs): 9.3
Total Number of Holdings: 469 Average Effective Duration (yrs): 5.8
Portfolio Turnover Rate (%): 45
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 1/31/26)
Portfolio structure
Fixed Income 96.0%
Money Market Funds 4.0%
Equities 0.0%
Issuer country weightings
Mexico 5.9%
United States 5.6%
Chile 4.5%
Saudi Arabia 4.3%
Argentina 3.7%
India 3.6%
Romania 3.5%
Colombia 3.5%
Turkey 3.5%
Other Countries 61.9%
Composition including fixed income credit
quality
AA 2.7%
A 11.2%
BBB 29.3%
BB 30.1%
B 12.5%
CCC 7.6%
C 0.7%
U.S. Government 1.5%
Not Rated 0.4%
Non-Fixed Income 0.0%
Money Market Funds 4.0%
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.