| Form N-CSR Cover | 6 Months Ended |
| Oct. 31, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | MFS SERIES TRUST IX |
| Entity Central Index Key | 0000063075 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Oct. 31, 2024 |
| Shareholder Report | 6 Months Ended |
| Oct. 31, 2024 USD ($) Holding | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | MFS SERIES TRUST IX |
| Entity Central Index Key | 0000063075 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Oct. 31, 2024 |
| C000006711 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Corporate Bond Fund |
| Class Name | Class A |
| Trading Symbol | MFBFX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.76 % |
| Expenses Paid, Amount | $ 39 |
| Expense Ratio, Percent | 0.76% |
| Net Assets | $ 4,662,201,140 |
| Holdings Count | Holding | 407 |
| Investment Company Portfolio Turnover | 19% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 4,662,201,140 Average Effective Maturity (yrs): 10.1 Total Number of Holdings: 407 Average Effective Duration (yrs): 6.8 Portfolio Turnover Rate (%): 19 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 10/31/24) Portfolio structure Fixed Income 97.3 % Money Market Funds 2.7 % Composition including fixed income credit quality AAA 3.3 % AA 3.7 % A 29.2 % BBB 49.3 % BB 6.8 % B 1.7 % C 0.0 % U.S. Government 3.3 % Not Rated 0.0 % Money Market Funds 2.7 % 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. |
| C000006716 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Corporate Bond Fund |
| Class Name | Class B |
| Trading Symbol | MFBBX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.51 % |
| Expenses Paid, Amount | $ 78 |
| Expense Ratio, Percent | 1.51% |
| Net Assets | $ 4,662,201,140 |
| Holdings Count | Holding | 407 |
| Investment Company Portfolio Turnover | 19% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 4,662,201,140 Average Effective Maturity (yrs): 10.1 Total Number of Holdings: 407 Average Effective Duration (yrs): 6.8 Portfolio Turnover Rate (%): 19 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 10/31/24) Portfolio structure Fixed Income 97.3 % Money Market Funds 2.7 % Composition including fixed income credit quality AAA 3.3 % AA 3.7 % A 29.2 % BBB 49.3 % BB 6.8 % B 1.7 % C 0.0 % U.S. Government 3.3 % Not Rated 0.0 % Money Market Funds 2.7 % 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. |
| C000006717 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Corporate Bond Fund |
| Class Name | Class C |
| Trading Symbol | MFBCX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.51 % |
| Expenses Paid, Amount | $ 78 |
| Expense Ratio, Percent | 1.51% |
| Net Assets | $ 4,662,201,140 |
| Holdings Count | Holding | 407 |
| Investment Company Portfolio Turnover | 19% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 4,662,201,140 Average Effective Maturity (yrs): 10.1 Total Number of Holdings: 407 Average Effective Duration (yrs): 6.8 Portfolio Turnover Rate (%): 19 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 10/31/24) Portfolio structure Fixed Income 97.3 % Money Market Funds 2.7 % Composition including fixed income credit quality AAA 3.3 % AA 3.7 % A 29.2 % BBB 49.3 % BB 6.8 % B 1.7 % C 0.0 % U.S. Government 3.3 % Not Rated 0.0 % Money Market Funds 2.7 % 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. |
| C000006718 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Corporate Bond Fund |
| Class Name | Class I |
| Trading Symbol | MBDIX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.51 % |
| Expenses Paid, Amount | $ 26 |
| Expense Ratio, Percent | 0.51% |
| Net Assets | $ 4,662,201,140 |
| Holdings Count | Holding | 407 |
| Investment Company Portfolio Turnover | 19% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 4,662,201,140 Average Effective Maturity (yrs): 10.1 Total Number of Holdings: 407 Average Effective Duration (yrs): 6.8 Portfolio Turnover Rate (%): 19 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 10/31/24) Portfolio structure Fixed Income 97.3 % Money Market Funds 2.7 % Composition including fixed income credit quality AAA 3.3 % AA 3.7 % A 29.2 % BBB 49.3 % BB 6.8 % B 1.7 % C 0.0 % U.S. Government 3.3 % Not Rated 0.0 % Money Market Funds 2.7 % 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. |
| C000006720 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Corporate Bond Fund |
| Class Name | Class R1 |
| Trading Symbol | MFBGX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.51 % |
| Expenses Paid, Amount | $ 78 |
| Expense Ratio, Percent | 1.51% |
| Net Assets | $ 4,662,201,140 |
| Holdings Count | Holding | 407 |
| Investment Company Portfolio Turnover | 19% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 4,662,201,140 Average Effective Maturity (yrs): 10.1 Total Number of Holdings: 407 Average Effective Duration (yrs): 6.8 Portfolio Turnover Rate (%): 19 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 10/31/24) Portfolio structure Fixed Income 97.3 % Money Market Funds 2.7 % Composition including fixed income credit quality AAA 3.3 % AA 3.7 % A 29.2 % BBB 49.3 % BB 6.8 % B 1.7 % C 0.0 % U.S. Government 3.3 % Not Rated 0.0 % Money Market Funds 2.7 % 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. |
| C000006722 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Corporate Bond Fund |
| Class Name | Class R2 |
| Trading Symbol | MBRRX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.01 % |
| Expenses Paid, Amount | $ 52 |
| Expense Ratio, Percent | 1.01% |
| Net Assets | $ 4,662,201,140 |
| Holdings Count | Holding | 407 |
| Investment Company Portfolio Turnover | 19% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 4,662,201,140 Average Effective Maturity (yrs): 10.1 Total Number of Holdings: 407 Average Effective Duration (yrs): 6.8 Portfolio Turnover Rate (%): 19 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 10/31/24) Portfolio structure Fixed Income 97.3 % Money Market Funds 2.7 % Composition including fixed income credit quality AAA 3.3 % AA 3.7 % A 29.2 % BBB 49.3 % BB 6.8 % B 1.7 % C 0.0 % U.S. Government 3.3 % Not Rated 0.0 % Money Market Funds 2.7 % 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. |
| C000006723 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Corporate Bond Fund |
| Class Name | Class R3 |
| Trading Symbol | MFBHX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.76 % |
| Expenses Paid, Amount | $ 39 |
| Expense Ratio, Percent | 0.76% |
| Net Assets | $ 4,662,201,140 |
| Holdings Count | Holding | 407 |
| Investment Company Portfolio Turnover | 19% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 4,662,201,140 Average Effective Maturity (yrs): 10.1 Total Number of Holdings: 407 Average Effective Duration (yrs): 6.8 Portfolio Turnover Rate (%): 19 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 10/31/24) Portfolio structure Fixed Income 97.3 % Money Market Funds 2.7 % Composition including fixed income credit quality AAA 3.3 % AA 3.7 % A 29.2 % BBB 49.3 % BB 6.8 % B 1.7 % C 0.0 % U.S. Government 3.3 % Not Rated 0.0 % Money Market Funds 2.7 % 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. |
| C000006712 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Corporate Bond Fund |
| Class Name | Class R4 |
| Trading Symbol | MFBJX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.51 % |
| Expenses Paid, Amount | $ 26 |
| Expense Ratio, Percent | 0.51% |
| Net Assets | $ 4,662,201,140 |
| Holdings Count | Holding | 407 |
| Investment Company Portfolio Turnover | 19% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 4,662,201,140 Average Effective Maturity (yrs): 10.1 Total Number of Holdings: 407 Average Effective Duration (yrs): 6.8 Portfolio Turnover Rate (%): 19 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 10/31/24) Portfolio structure Fixed Income 97.3 % Money Market Funds 2.7 % Composition including fixed income credit quality AAA 3.3 % AA 3.7 % A 29.2 % BBB 49.3 % BB 6.8 % B 1.7 % C 0.0 % U.S. Government 3.3 % Not Rated 0.0 % Money Market Funds 2.7 % 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. |
| C000070424 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Corporate Bond Fund |
| Class Name | Class R6 |
| Trading Symbol | MFBKX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.42 % |
| Expenses Paid, Amount | $ 22 |
| Expense Ratio, Percent | 0.42% |
| Net Assets | $ 4,662,201,140 |
| Holdings Count | Holding | 407 |
| Investment Company Portfolio Turnover | 19% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 4,662,201,140 Average Effective Maturity (yrs): 10.1 Total Number of Holdings: 407 Average Effective Duration (yrs): 6.8 Portfolio Turnover Rate (%): 19 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 10/31/24) Portfolio structure Fixed Income 97.3 % Money Market Funds 2.7 % Composition including fixed income credit quality AAA 3.3 % AA 3.7 % A 29.2 % BBB 49.3 % BB 6.8 % B 1.7 % C 0.0 % U.S. Government 3.3 % Not Rated 0.0 % Money Market Funds 2.7 % 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. |
| C000006745 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Limited Maturity Fund |
| Class Name | Class A |
| Trading Symbol | MQLFX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.57 % |
| Expenses Paid, Amount | $ 29 |
| Expense Ratio, Percent | 0.57% |
| Net Assets | $ 2,958,033,458 |
| Holdings Count | Holding | 372 |
| Investment Company Portfolio Turnover | 20% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,958,033,458 Average Effective Maturity (yrs): 2.1 Total Number of Holdings: 372 Average Effective Duration (yrs): 1.8 Portfolio Turnover Rate (%): 20 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 10/31/24) Portfolio structure Fixed Income 97.6 % Money Market Funds 2.4 % Composition including fixed income credit quality AAA 10.6 % AA 8.4 % A 18.5 % BBB 31.9 % BB 0.0 % CC 0.0 % C 0.0 % U.S. Government 27.7 % Federal Agencies 0.5 % Not Rated 0.0 % Money Market Funds 2.4 % 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. |
| C000006750 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Limited Maturity Fund |
| Class Name | Class B |
| Trading Symbol | MQLBX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.32 % |
| Expenses Paid, Amount | $ 68 |
| Expense Ratio, Percent | 1.32% |
| Net Assets | $ 2,958,033,458 |
| Holdings Count | Holding | 372 |
| Investment Company Portfolio Turnover | 20% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,958,033,458 Average Effective Maturity (yrs): 2.1 Total Number of Holdings: 372 Average Effective Duration (yrs): 1.8 Portfolio Turnover Rate (%): 20 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 10/31/24) Portfolio structure Fixed Income 97.6 % Money Market Funds 2.4 % Composition including fixed income credit quality AAA 10.6 % AA 8.4 % A 18.5 % BBB 31.9 % BB 0.0 % CC 0.0 % C 0.0 % U.S. Government 27.7 % Federal Agencies 0.5 % Not Rated 0.0 % Money Market Funds 2.4 % 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. |
| C000006751 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Limited Maturity Fund |
| Class Name | Class C |
| Trading Symbol | MQLCX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.42 % |
| Expenses Paid, Amount | $ 73 |
| Expense Ratio, Percent | 1.42% |
| Net Assets | $ 2,958,033,458 |
| Holdings Count | Holding | 372 |
| Investment Company Portfolio Turnover | 20% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,958,033,458 Average Effective Maturity (yrs): 2.1 Total Number of Holdings: 372 Average Effective Duration (yrs): 1.8 Portfolio Turnover Rate (%): 20 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 10/31/24) Portfolio structure Fixed Income 97.6 % Money Market Funds 2.4 % Composition including fixed income credit quality AAA 10.6 % AA 8.4 % A 18.5 % BBB 31.9 % BB 0.0 % CC 0.0 % C 0.0 % U.S. Government 27.7 % Federal Agencies 0.5 % Not Rated 0.0 % Money Market Funds 2.4 % 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. |
| C000006744 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Limited Maturity Fund |
| Class Name | Class I |
| Trading Symbol | MQLIX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.42 % |
| Expenses Paid, Amount | $ 22 |
| Expense Ratio, Percent | 0.42% |
| Net Assets | $ 2,958,033,458 |
| Holdings Count | Holding | 372 |
| Investment Company Portfolio Turnover | 20% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,958,033,458 Average Effective Maturity (yrs): 2.1 Total Number of Holdings: 372 Average Effective Duration (yrs): 1.8 Portfolio Turnover Rate (%): 20 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 10/31/24) Portfolio structure Fixed Income 97.6 % Money Market Funds 2.4 % Composition including fixed income credit quality AAA 10.6 % AA 8.4 % A 18.5 % BBB 31.9 % BB 0.0 % CC 0.0 % C 0.0 % U.S. Government 27.7 % Federal Agencies 0.5 % Not Rated 0.0 % Money Market Funds 2.4 % 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. |
| C000006753 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Limited Maturity Fund |
| Class Name | Class R1 |
| Trading Symbol | MQLGX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.42 % |
| Expenses Paid, Amount | $ 73 |
| Expense Ratio, Percent | 1.42% |
| Net Assets | $ 2,958,033,458 |
| Holdings Count | Holding | 372 |
| Investment Company Portfolio Turnover | 20% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,958,033,458 Average Effective Maturity (yrs): 2.1 Total Number of Holdings: 372 Average Effective Duration (yrs): 1.8 Portfolio Turnover Rate (%): 20 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 10/31/24) Portfolio structure Fixed Income 97.6 % Money Market Funds 2.4 % Composition including fixed income credit quality AAA 10.6 % AA 8.4 % A 18.5 % BBB 31.9 % BB 0.0 % CC 0.0 % C 0.0 % U.S. Government 27.7 % Federal Agencies 0.5 % Not Rated 0.0 % Money Market Funds 2.4 % 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. |
| C000006755 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Limited Maturity Fund |
| Class Name | Class R2 |
| Trading Symbol | MLMRX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.82 % |
| Expenses Paid, Amount | $ 42 |
| Expense Ratio, Percent | 0.82% |
| Net Assets | $ 2,958,033,458 |
| Holdings Count | Holding | 372 |
| Investment Company Portfolio Turnover | 20% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,958,033,458 Average Effective Maturity (yrs): 2.1 Total Number of Holdings: 372 Average Effective Duration (yrs): 1.8 Portfolio Turnover Rate (%): 20 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 10/31/24) Portfolio structure Fixed Income 97.6 % Money Market Funds 2.4 % Composition including fixed income credit quality AAA 10.6 % AA 8.4 % A 18.5 % BBB 31.9 % BB 0.0 % CC 0.0 % C 0.0 % U.S. Government 27.7 % Federal Agencies 0.5 % Not Rated 0.0 % Money Market Funds 2.4 % 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. |
| C000006756 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Limited Maturity Fund |
| Class Name | Class R3 |
| Trading Symbol | MQLHX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.67 % |
| Expenses Paid, Amount | $ 34 |
| Expense Ratio, Percent | 0.67% |
| Net Assets | $ 2,958,033,458 |
| Holdings Count | Holding | 372 |
| Investment Company Portfolio Turnover | 20% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,958,033,458 Average Effective Maturity (yrs): 2.1 Total Number of Holdings: 372 Average Effective Duration (yrs): 1.8 Portfolio Turnover Rate (%): 20 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 10/31/24) Portfolio structure Fixed Income 97.6 % Money Market Funds 2.4 % Composition including fixed income credit quality AAA 10.6 % AA 8.4 % A 18.5 % BBB 31.9 % BB 0.0 % CC 0.0 % C 0.0 % U.S. Government 27.7 % Federal Agencies 0.5 % Not Rated 0.0 % Money Market Funds 2.4 % 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. |
| C000006746 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Limited Maturity Fund |
| Class Name | Class R4 |
| Trading Symbol | MQLJX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.42 % |
| Expenses Paid, Amount | $ 22 |
| Expense Ratio, Percent | 0.42% |
| Net Assets | $ 2,958,033,458 |
| Holdings Count | Holding | 372 |
| Investment Company Portfolio Turnover | 20% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,958,033,458 Average Effective Maturity (yrs): 2.1 Total Number of Holdings: 372 Average Effective Duration (yrs): 1.8 Portfolio Turnover Rate (%): 20 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 10/31/24) Portfolio structure Fixed Income 97.6 % Money Market Funds 2.4 % Composition including fixed income credit quality AAA 10.6 % AA 8.4 % A 18.5 % BBB 31.9 % BB 0.0 % CC 0.0 % C 0.0 % U.S. Government 27.7 % Federal Agencies 0.5 % Not Rated 0.0 % Money Market Funds 2.4 % 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. |
| C000119776 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Limited Maturity Fund |
| Class Name | Class R6 |
| Trading Symbol | MQLKX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.35 % |
| Expenses Paid, Amount | $ 18 |
| Expense Ratio, Percent | 0.35% |
| Net Assets | $ 2,958,033,458 |
| Holdings Count | Holding | 372 |
| Investment Company Portfolio Turnover | 20% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,958,033,458 Average Effective Maturity (yrs): 2.1 Total Number of Holdings: 372 Average Effective Duration (yrs): 1.8 Portfolio Turnover Rate (%): 20 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 10/31/24) Portfolio structure Fixed Income 97.6 % Money Market Funds 2.4 % Composition including fixed income credit quality AAA 10.6 % AA 8.4 % A 18.5 % BBB 31.9 % BB 0.0 % CC 0.0 % C 0.0 % U.S. Government 27.7 % Federal Agencies 0.5 % Not Rated 0.0 % Money Market Funds 2.4 % 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. |
| C000006757 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Municipal Limited Maturity Fund |
| Class Name | Class A |
| Trading Symbol | MTLFX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.59 % |
| Expenses Paid, Amount | $ 30 |
| Expense Ratio, Percent | 0.59% |
| Net Assets | $ 2,561,451,049 |
| Holdings Count | Holding | 1,209 |
| Investment Company Portfolio Turnover | 13% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,561,451,049 Average Effective Maturity (yrs): 4.7 Total Number of Holdings: 1,209 Average Effective Duration (yrs): 3.3 Portfolio Turnover Rate (%): 13 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 10/31/24) Portfolio structure Fixed Income 98.7 % Money Market Funds 1.3 % Composition including fixed income credit quality AAA 11.1 % AA 26.0 % A 32.3 % BBB 18.2 % BB 5.1 % B 0.2 % CC 0.0 % D 0.0 % Not Rated 5.8 % Money Market Funds 1.3 % 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. |
| C000006758 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Municipal Limited Maturity Fund |
| Class Name | Class B |
| Trading Symbol | MTLBX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.33 % |
| Expenses Paid, Amount | $ 68 |
| Expense Ratio, Percent | 1.33% |
| Net Assets | $ 2,561,451,049 |
| Holdings Count | Holding | 1,209 |
| Investment Company Portfolio Turnover | 13% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,561,451,049 Average Effective Maturity (yrs): 4.7 Total Number of Holdings: 1,209 Average Effective Duration (yrs): 3.3 Portfolio Turnover Rate (%): 13 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 10/31/24) Portfolio structure Fixed Income 98.7 % Money Market Funds 1.3 % Composition including fixed income credit quality AAA 11.1 % AA 26.0 % A 32.3 % BBB 18.2 % BB 5.1 % B 0.2 % CC 0.0 % D 0.0 % Not Rated 5.8 % Money Market Funds 1.3 % 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. |
| C000006759 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Municipal Limited Maturity Fund |
| Class Name | Class C |
| Trading Symbol | MTLCX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.43 % |
| Expenses Paid, Amount | $ 73 |
| Expense Ratio, Percent | 1.43% |
| Net Assets | $ 2,561,451,049 |
| Holdings Count | Holding | 1,209 |
| Investment Company Portfolio Turnover | 13% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,561,451,049 Average Effective Maturity (yrs): 4.7 Total Number of Holdings: 1,209 Average Effective Duration (yrs): 3.3 Portfolio Turnover Rate (%): 13 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 10/31/24) Portfolio structure Fixed Income 98.7 % Money Market Funds 1.3 % Composition including fixed income credit quality AAA 11.1 % AA 26.0 % A 32.3 % BBB 18.2 % BB 5.1 % B 0.2 % CC 0.0 % D 0.0 % Not Rated 5.8 % Money Market Funds 1.3 % 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. |
| C000092113 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Municipal Limited Maturity Fund |
| Class Name | Class I |
| Trading Symbol | MTLIX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.44 % |
| Expenses Paid, Amount | $ 22 |
| Expense Ratio, Percent | 0.44% |
| Net Assets | $ 2,561,451,049 |
| Holdings Count | Holding | 1,209 |
| Investment Company Portfolio Turnover | 13% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,561,451,049 Average Effective Maturity (yrs): 4.7 Total Number of Holdings: 1,209 Average Effective Duration (yrs): 3.3 Portfolio Turnover Rate (%): 13 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 10/31/24) Portfolio structure Fixed Income 98.7 % Money Market Funds 1.3 % Composition including fixed income credit quality AAA 11.1 % AA 26.0 % A 32.3 % BBB 18.2 % BB 5.1 % B 0.2 % CC 0.0 % D 0.0 % Not Rated 5.8 % Money Market Funds 1.3 % 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. |
| C000195152 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Municipal Limited Maturity Fund |
| Class Name | Class R6 |
| Trading Symbol | MTLRX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.36 % |
| Expenses Paid, Amount | $ 18 |
| Expense Ratio, Percent | 0.36% |
| Net Assets | $ 2,561,451,049 |
| Holdings Count | Holding | 1,209 |
| Investment Company Portfolio Turnover | 13% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 2,561,451,049 Average Effective Maturity (yrs): 4.7 Total Number of Holdings: 1,209 Average Effective Duration (yrs): 3.3 Portfolio Turnover Rate (%): 13 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 10/31/24) Portfolio structure Fixed Income 98.7 % Money Market Funds 1.3 % Composition including fixed income credit quality AAA 11.1 % AA 26.0 % A 32.3 % BBB 18.2 % BB 5.1 % B 0.2 % CC 0.0 % D 0.0 % Not Rated 5.8 % Money Market Funds 1.3 % 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. |
| C000006760 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Total Return Bond Fund |
| Class Name | Class A |
| Trading Symbol | MRBFX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.60 % |
| Expenses Paid, Amount | $ 31 |
| Expense Ratio, Percent | 0.60% |
| Net Assets | $ 8,102,089,007 |
| Holdings Count | Holding | 1,134 |
| Investment Company Portfolio Turnover | 45% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 8,102,089,007 Average Effective Maturity (yrs): 7.8 Total Number of Holdings: 1,134 Average Effective Duration (yrs): 6.1 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 10/31/24) Portfolio structure Fixed Income 97.4 % Money Market Funds 2.6 % Composition including fixed income credit quality AAA 9.9 % AA 4.9 % A 10.4 % BBB 22.4 % BB 4.4 % B 1.5 % CCC 0.0 % CC 0.0 % C 0.0 % U.S. Government 23.5 % Federal Agencies 20.1 % Not Rated 0.3 % Money Market Funds 2.6 % 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. |
| C000006765 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Total Return Bond Fund |
| Class Name | Class B |
| Trading Symbol | MRBBX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.35 % |
| Expenses Paid, Amount | $ 70 |
| Expense Ratio, Percent | 1.35% |
| Net Assets | $ 8,102,089,007 |
| Holdings Count | Holding | 1,134 |
| Investment Company Portfolio Turnover | 45% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 8,102,089,007 Average Effective Maturity (yrs): 7.8 Total Number of Holdings: 1,134 Average Effective Duration (yrs): 6.1 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 10/31/24) Portfolio structure Fixed Income 97.4 % Money Market Funds 2.6 % Composition including fixed income credit quality AAA 9.9 % AA 4.9 % A 10.4 % BBB 22.4 % BB 4.4 % B 1.5 % CCC 0.0 % CC 0.0 % C 0.0 % U.S. Government 23.5 % Federal Agencies 20.1 % Not Rated 0.3 % Money Market Funds 2.6 % 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. |
| C000006766 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Total Return Bond Fund |
| Class Name | Class C |
| Trading Symbol | MRBCX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.45 % |
| Expenses Paid, Amount | $ 75 |
| Expense Ratio, Percent | 1.45% |
| Net Assets | $ 8,102,089,007 |
| Holdings Count | Holding | 1,134 |
| Investment Company Portfolio Turnover | 45% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 8,102,089,007 Average Effective Maturity (yrs): 7.8 Total Number of Holdings: 1,134 Average Effective Duration (yrs): 6.1 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 10/31/24) Portfolio structure Fixed Income 97.4 % Money Market Funds 2.6 % Composition including fixed income credit quality AAA 9.9 % AA 4.9 % A 10.4 % BBB 22.4 % BB 4.4 % B 1.5 % CCC 0.0 % CC 0.0 % C 0.0 % U.S. Government 23.5 % Federal Agencies 20.1 % Not Rated 0.3 % Money Market Funds 2.6 % 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. |
| C000006767 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Total Return Bond Fund |
| Class Name | Class I |
| Trading Symbol | MRBIX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.45 % |
| Expenses Paid, Amount | $ 23 |
| Expense Ratio, Percent | 0.45% |
| Net Assets | $ 8,102,089,007 |
| Holdings Count | Holding | 1,134 |
| Investment Company Portfolio Turnover | 45% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 8,102,089,007 Average Effective Maturity (yrs): 7.8 Total Number of Holdings: 1,134 Average Effective Duration (yrs): 6.1 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 10/31/24) Portfolio structure Fixed Income 97.4 % Money Market Funds 2.6 % Composition including fixed income credit quality AAA 9.9 % AA 4.9 % A 10.4 % BBB 22.4 % BB 4.4 % B 1.5 % CCC 0.0 % CC 0.0 % C 0.0 % U.S. Government 23.5 % Federal Agencies 20.1 % Not Rated 0.3 % Money Market Funds 2.6 % 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. |
| C000006769 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Total Return Bond Fund |
| Class Name | Class R1 |
| Trading Symbol | MRBGX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 1.45 % |
| Expenses Paid, Amount | $ 75 |
| Expense Ratio, Percent | 1.45% |
| Net Assets | $ 8,102,089,007 |
| Holdings Count | Holding | 1,134 |
| Investment Company Portfolio Turnover | 45% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 8,102,089,007 Average Effective Maturity (yrs): 7.8 Total Number of Holdings: 1,134 Average Effective Duration (yrs): 6.1 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 10/31/24) Portfolio structure Fixed Income 97.4 % Money Market Funds 2.6 % Composition including fixed income credit quality AAA 9.9 % AA 4.9 % A 10.4 % BBB 22.4 % BB 4.4 % B 1.5 % CCC 0.0 % CC 0.0 % C 0.0 % U.S. Government 23.5 % Federal Agencies 20.1 % Not Rated 0.3 % Money Market Funds 2.6 % 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. |
| C000006771 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Total Return Bond Fund |
| Class Name | Class R2 |
| Trading Symbol | MRRRX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.95 % |
| Expenses Paid, Amount | $ 49 |
| Expense Ratio, Percent | 0.95% |
| Net Assets | $ 8,102,089,007 |
| Holdings Count | Holding | 1,134 |
| Investment Company Portfolio Turnover | 45% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 8,102,089,007 Average Effective Maturity (yrs): 7.8 Total Number of Holdings: 1,134 Average Effective Duration (yrs): 6.1 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 10/31/24) Portfolio structure Fixed Income 97.4 % Money Market Funds 2.6 % Composition including fixed income credit quality AAA 9.9 % AA 4.9 % A 10.4 % BBB 22.4 % BB 4.4 % B 1.5 % CCC 0.0 % CC 0.0 % C 0.0 % U.S. Government 23.5 % Federal Agencies 20.1 % Not Rated 0.3 % Money Market Funds 2.6 % 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. |
| C000006772 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Total Return Bond Fund |
| Class Name | Class R3 |
| Trading Symbol | MRBHX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.70 % |
| Expenses Paid, Amount | $ 36 |
| Expense Ratio, Percent | 0.70% |
| Net Assets | $ 8,102,089,007 |
| Holdings Count | Holding | 1,134 |
| Investment Company Portfolio Turnover | 45% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 8,102,089,007 Average Effective Maturity (yrs): 7.8 Total Number of Holdings: 1,134 Average Effective Duration (yrs): 6.1 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 10/31/24) Portfolio structure Fixed Income 97.4 % Money Market Funds 2.6 % Composition including fixed income credit quality AAA 9.9 % AA 4.9 % A 10.4 % BBB 22.4 % BB 4.4 % B 1.5 % CCC 0.0 % CC 0.0 % C 0.0 % U.S. Government 23.5 % Federal Agencies 20.1 % Not Rated 0.3 % Money Market Funds 2.6 % 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. |
| C000006761 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Total Return Bond Fund |
| Class Name | Class R4 |
| Trading Symbol | MRBJX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.45 % |
| Expenses Paid, Amount | $ 23 |
| Expense Ratio, Percent | 0.45% |
| Net Assets | $ 8,102,089,007 |
| Holdings Count | Holding | 1,134 |
| Investment Company Portfolio Turnover | 45% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 8,102,089,007 Average Effective Maturity (yrs): 7.8 Total Number of Holdings: 1,134 Average Effective Duration (yrs): 6.1 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 10/31/24) Portfolio structure Fixed Income 97.4 % Money Market Funds 2.6 % Composition including fixed income credit quality AAA 9.9 % AA 4.9 % A 10.4 % BBB 22.4 % BB 4.4 % B 1.5 % CCC 0.0 % CC 0.0 % C 0.0 % U.S. Government 23.5 % Federal Agencies 20.1 % Not Rated 0.3 % Money Market Funds 2.6 % 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. |
| C000034443 [Member] | |
| Shareholder Report [Line Items] | |
| Fund Name | MFS® Total Return Bond Fund |
| Class Name | Class R6 |
| Trading Symbol | MRBKX |
| Annual or Semi-Annual Statement [Text Block] | This semiannual shareholder report |
| 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 |
| 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 $ 0.36 % |
| Expenses Paid, Amount | $ 19 |
| Expense Ratio, Percent | 0.36% |
| Net Assets | $ 8,102,089,007 |
| Holdings Count | Holding | 1,134 |
| Investment Company Portfolio Turnover | 45% |
| Additional Fund Statistics [Text Block] | FUND STATISTICS AS OF 10/31/24 Net Assets ($): 8,102,089,007 Average Effective Maturity (yrs): 7.8 Total Number of Holdings: 1,134 Average Effective Duration (yrs): 6.1 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 10/31/24) Portfolio structure Fixed Income 97.4 % Money Market Funds 2.6 % Composition including fixed income credit quality AAA 9.9 % AA 4.9 % A 10.4 % BBB 22.4 % BB 4.4 % B 1.5 % CCC 0.0 % CC 0.0 % C 0.0 % U.S. Government 23.5 % Federal Agencies 20.1 % Not Rated 0.3 % Money Market Funds 2.6 % 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. |
| Shareholder Report, Holdings (Details) | Oct. 31, 2024 |
| C000006711 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.70% |
| C000006711 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 29.20% |
| C000006711 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 49.30% |
| C000006711 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 6.80% |
| C000006711 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.70% |
| C000006711 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006711 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006711 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006711 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006711 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.30% |
| C000006711 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006711 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006716 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.70% |
| C000006716 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 29.20% |
| C000006716 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 49.30% |
| C000006716 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 6.80% |
| C000006716 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.70% |
| C000006716 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006716 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006716 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006716 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006716 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.30% |
| C000006716 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006716 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006717 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.70% |
| C000006717 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 29.20% |
| C000006717 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 49.30% |
| C000006717 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 6.80% |
| C000006717 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.70% |
| C000006717 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006717 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006717 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006717 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006717 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.30% |
| C000006717 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006717 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006718 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.70% |
| C000006718 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 29.20% |
| C000006718 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 49.30% |
| C000006718 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 6.80% |
| C000006718 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.70% |
| C000006718 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006718 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006718 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006718 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006718 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.30% |
| C000006718 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006718 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006720 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.70% |
| C000006720 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 29.20% |
| C000006720 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 49.30% |
| C000006720 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 6.80% |
| C000006720 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.70% |
| C000006720 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006720 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006720 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006720 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006720 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.30% |
| C000006720 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006720 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006722 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.70% |
| C000006722 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 29.20% |
| C000006722 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 49.30% |
| C000006722 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 6.80% |
| C000006722 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.70% |
| C000006722 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006722 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006722 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006722 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006722 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.30% |
| C000006722 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006722 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006723 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.70% |
| C000006723 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 29.20% |
| C000006723 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 49.30% |
| C000006723 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 6.80% |
| C000006723 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.70% |
| C000006723 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006723 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006723 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006723 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006723 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.30% |
| C000006723 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006723 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006712 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.70% |
| C000006712 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 29.20% |
| C000006712 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 49.30% |
| C000006712 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 6.80% |
| C000006712 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.70% |
| C000006712 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006712 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006712 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006712 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006712 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.30% |
| C000006712 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000006712 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000070424 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.70% |
| C000070424 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 29.20% |
| C000070424 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 49.30% |
| C000070424 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 6.80% |
| C000070424 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.70% |
| C000070424 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000070424 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000070424 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000070424 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000070424 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.30% |
| C000070424 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.70% |
| C000070424 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 3.30% |
| C000006745 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 8.40% |
| C000006745 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.50% |
| C000006745 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 31.90% |
| C000006745 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006745 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006745 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006745 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006745 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.60% |
| C000006745 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006745 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.60% |
| C000006745 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006745 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 27.70% |
| C000006745 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.50% |
| C000006750 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 8.40% |
| C000006750 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.50% |
| C000006750 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 31.90% |
| C000006750 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006750 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006750 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006750 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006750 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.60% |
| C000006750 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006750 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.60% |
| C000006750 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006750 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 27.70% |
| C000006750 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.50% |
| C000006751 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 8.40% |
| C000006751 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.50% |
| C000006751 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 31.90% |
| C000006751 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006751 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006751 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006751 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006751 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.60% |
| C000006751 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006751 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.60% |
| C000006751 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006751 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 27.70% |
| C000006751 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.50% |
| C000006744 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 8.40% |
| C000006744 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.50% |
| C000006744 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 31.90% |
| C000006744 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006744 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006744 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006744 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006744 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.60% |
| C000006744 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006744 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.60% |
| C000006744 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006744 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 27.70% |
| C000006744 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.50% |
| C000006753 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 8.40% |
| C000006753 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.50% |
| C000006753 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 31.90% |
| C000006753 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006753 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006753 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006753 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006753 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.60% |
| C000006753 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006753 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.60% |
| C000006753 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006753 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 27.70% |
| C000006753 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.50% |
| C000006755 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 8.40% |
| C000006755 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.50% |
| C000006755 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 31.90% |
| C000006755 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006755 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006755 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006755 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006755 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.60% |
| C000006755 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006755 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.60% |
| C000006755 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006755 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 27.70% |
| C000006755 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.50% |
| C000006756 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 8.40% |
| C000006756 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.50% |
| C000006756 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 31.90% |
| C000006756 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006756 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006756 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006756 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006756 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.60% |
| C000006756 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006756 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.60% |
| C000006756 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006756 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 27.70% |
| C000006756 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.50% |
| C000006746 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 8.40% |
| C000006746 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.50% |
| C000006746 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 31.90% |
| C000006746 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006746 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006746 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006746 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006746 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.60% |
| C000006746 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006746 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.60% |
| C000006746 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000006746 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 27.70% |
| C000006746 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.50% |
| C000119776 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 8.40% |
| C000119776 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.50% |
| C000119776 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 31.90% |
| C000119776 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000119776 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000119776 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000119776 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000119776 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.60% |
| C000119776 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000119776 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.60% |
| C000119776 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.40% |
| C000119776 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 27.70% |
| C000119776 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.50% |
| C000006757 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 26% |
| C000006757 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 32.30% |
| C000006757 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.20% |
| C000006757 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.10% |
| C000006757 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.20% |
| C000006757 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006757 [Member] | Fitch, D Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006757 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.80% |
| C000006757 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 11.10% |
| C000006757 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000006757 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 98.70% |
| C000006757 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000006758 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 26% |
| C000006758 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 32.30% |
| C000006758 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.20% |
| C000006758 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.10% |
| C000006758 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.20% |
| C000006758 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006758 [Member] | Fitch, D Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006758 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.80% |
| C000006758 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 11.10% |
| C000006758 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000006758 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 98.70% |
| C000006758 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000006759 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 26% |
| C000006759 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 32.30% |
| C000006759 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.20% |
| C000006759 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.10% |
| C000006759 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.20% |
| C000006759 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006759 [Member] | Fitch, D Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006759 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.80% |
| C000006759 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 11.10% |
| C000006759 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000006759 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 98.70% |
| C000006759 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000092113 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 26% |
| C000092113 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 32.30% |
| C000092113 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.20% |
| C000092113 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.10% |
| C000092113 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.20% |
| C000092113 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000092113 [Member] | Fitch, D Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000092113 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.80% |
| C000092113 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 11.10% |
| C000092113 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000092113 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 98.70% |
| C000092113 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000195152 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 26% |
| C000195152 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 32.30% |
| C000195152 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 18.20% |
| C000195152 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.10% |
| C000195152 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.20% |
| C000195152 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000195152 [Member] | Fitch, D Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000195152 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 5.80% |
| C000195152 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 11.10% |
| C000195152 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000195152 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 98.70% |
| C000195152 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.30% |
| C000006760 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.90% |
| C000006760 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.40% |
| C000006760 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 22.40% |
| C000006760 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.40% |
| C000006760 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.50% |
| C000006760 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006760 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006760 [Member] | Fitch, CCC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006760 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.30% |
| C000006760 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 9.90% |
| C000006760 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006760 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.40% |
| C000006760 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006760 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 23.50% |
| C000006760 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 20.10% |
| C000006765 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.90% |
| C000006765 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.40% |
| C000006765 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 22.40% |
| C000006765 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.40% |
| C000006765 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.50% |
| C000006765 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006765 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006765 [Member] | Fitch, CCC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006765 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.30% |
| C000006765 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 9.90% |
| C000006765 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006765 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.40% |
| C000006765 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006765 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 23.50% |
| C000006765 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 20.10% |
| C000006766 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.90% |
| C000006766 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.40% |
| C000006766 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 22.40% |
| C000006766 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.40% |
| C000006766 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.50% |
| C000006766 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006766 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006766 [Member] | Fitch, CCC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006766 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.30% |
| C000006766 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 9.90% |
| C000006766 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006766 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.40% |
| C000006766 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006766 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 23.50% |
| C000006766 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 20.10% |
| C000006767 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.90% |
| C000006767 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.40% |
| C000006767 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 22.40% |
| C000006767 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.40% |
| C000006767 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.50% |
| C000006767 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006767 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006767 [Member] | Fitch, CCC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006767 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.30% |
| C000006767 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 9.90% |
| C000006767 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006767 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.40% |
| C000006767 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006767 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 23.50% |
| C000006767 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 20.10% |
| C000006769 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.90% |
| C000006769 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.40% |
| C000006769 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 22.40% |
| C000006769 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.40% |
| C000006769 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.50% |
| C000006769 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006769 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006769 [Member] | Fitch, CCC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006769 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.30% |
| C000006769 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 9.90% |
| C000006769 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006769 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.40% |
| C000006769 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006769 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 23.50% |
| C000006769 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 20.10% |
| C000006771 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.90% |
| C000006771 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.40% |
| C000006771 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 22.40% |
| C000006771 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.40% |
| C000006771 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.50% |
| C000006771 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006771 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006771 [Member] | Fitch, CCC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006771 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.30% |
| C000006771 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 9.90% |
| C000006771 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006771 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.40% |
| C000006771 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006771 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 23.50% |
| C000006771 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 20.10% |
| C000006772 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.90% |
| C000006772 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.40% |
| C000006772 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 22.40% |
| C000006772 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.40% |
| C000006772 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.50% |
| C000006772 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006772 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006772 [Member] | Fitch, CCC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006772 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.30% |
| C000006772 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 9.90% |
| C000006772 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006772 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.40% |
| C000006772 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006772 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 23.50% |
| C000006772 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 20.10% |
| C000006761 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.90% |
| C000006761 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.40% |
| C000006761 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 22.40% |
| C000006761 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.40% |
| C000006761 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.50% |
| C000006761 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006761 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006761 [Member] | Fitch, CCC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000006761 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.30% |
| C000006761 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 9.90% |
| C000006761 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006761 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.40% |
| C000006761 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000006761 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 23.50% |
| C000006761 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 20.10% |
| C000034443 [Member] | Fitch, AA Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.90% |
| C000034443 [Member] | Fitch, A Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 10.40% |
| C000034443 [Member] | Fitch, BBB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 22.40% |
| C000034443 [Member] | Fitch, BB Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 4.40% |
| C000034443 [Member] | Fitch, B Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 1.50% |
| C000034443 [Member] | Fitch, C Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000034443 [Member] | Fitch, CC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000034443 [Member] | Fitch, CCC Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0% |
| C000034443 [Member] | Not Rated [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 0.30% |
| C000034443 [Member] | Moody's, Aaa Rating [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 9.90% |
| C000034443 [Member] | Money Market Funds Quality [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000034443 [Member] | Fixed Income Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 97.40% |
| C000034443 [Member] | Money Market Funds [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 2.60% |
| C000034443 [Member] | US Government Debt Securities [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 23.50% |
| C000034443 [Member] | Federal Agencies [Member] | |
| Holdings [Line Items] | |
| Percent of Total Investments | 20.10% |
