Shareholder Report, Holdings (Details)
|
Apr. 30, 2026 |
| C000006757 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
34.40%
|
| C000006757 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.60%
|
| C000006757 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
37.00%
|
| C000006757 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.50%
|
| C000006757 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.30%
|
| C000006757 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000006757 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006757 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.90%
|
| C000006757 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000006757 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
96.40%
|
| C000006758 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
34.40%
|
| C000006758 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.60%
|
| C000006758 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
37.00%
|
| C000006758 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.50%
|
| C000006758 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.30%
|
| C000006758 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000006758 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006758 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.90%
|
| C000006758 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000006758 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
96.40%
|
| C000006759 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
34.40%
|
| C000006759 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.60%
|
| C000006759 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
37.00%
|
| C000006759 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.50%
|
| C000006759 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.30%
|
| C000006759 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000006759 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006759 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.90%
|
| C000006759 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000006759 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
96.40%
|
| C000092113 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
34.40%
|
| C000092113 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.60%
|
| C000092113 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
37.00%
|
| C000092113 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.50%
|
| C000092113 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.30%
|
| C000092113 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000092113 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000092113 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.90%
|
| C000092113 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000092113 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
96.40%
|
| C000195152 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
34.40%
|
| C000195152 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.60%
|
| C000195152 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
37.00%
|
| C000195152 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.50%
|
| C000195152 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.30%
|
| C000195152 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000195152 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000195152 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.90%
|
| C000195152 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.60%
|
| C000195152 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
96.40%
|
| C000006760 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006760 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006760 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
10.80%
|
| C000006760 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.00%
|
| C000006760 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006760 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006760 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006760 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006760 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.70%
|
| C000006760 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006760 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
24.00%
|
| C000006760 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006760 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
14.70%
|
| C000006760 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.50%
|
| C000006765 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006765 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006765 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
10.80%
|
| C000006765 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.00%
|
| C000006765 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006765 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006765 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006765 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006765 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.70%
|
| C000006765 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006765 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
24.00%
|
| C000006765 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006765 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
14.70%
|
| C000006765 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.50%
|
| C000006766 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006766 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006766 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
10.80%
|
| C000006766 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.00%
|
| C000006766 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006766 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006766 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006766 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006766 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.70%
|
| C000006766 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006766 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
24.00%
|
| C000006766 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006766 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
14.70%
|
| C000006766 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.50%
|
| C000006767 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006767 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006767 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
10.80%
|
| C000006767 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.00%
|
| C000006767 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006767 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006767 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006767 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006767 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.70%
|
| C000006767 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006767 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
24.00%
|
| C000006767 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006767 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
14.70%
|
| C000006767 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.50%
|
| C000006769 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006769 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006769 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
10.80%
|
| C000006769 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.00%
|
| C000006769 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006769 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006769 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006769 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006769 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.70%
|
| C000006769 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006769 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
24.00%
|
| C000006769 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006769 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
14.70%
|
| C000006769 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.50%
|
| C000006771 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006771 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006771 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
10.80%
|
| C000006771 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.00%
|
| C000006771 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006771 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006771 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006771 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006771 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.70%
|
| C000006771 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006771 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
24.00%
|
| C000006771 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006771 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
14.70%
|
| C000006771 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.50%
|
| C000006772 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006772 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006772 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
10.80%
|
| C000006772 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.00%
|
| C000006772 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006772 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006772 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006772 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006772 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.70%
|
| C000006772 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006772 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
24.00%
|
| C000006772 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006772 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
14.70%
|
| C000006772 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.50%
|
| C000006761 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006761 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006761 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
10.80%
|
| C000006761 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.00%
|
| C000006761 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006761 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006761 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006761 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006761 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.70%
|
| C000006761 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006761 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
24.00%
|
| C000006761 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000006761 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
14.70%
|
| C000006761 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.50%
|
| C000034443 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000034443 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000034443 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
10.80%
|
| C000034443 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
4.00%
|
| C000034443 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000034443 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000034443 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000034443 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000034443 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
13.70%
|
| C000034443 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000034443 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
24.00%
|
| C000034443 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.50%
|
| C000034443 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
14.70%
|
| C000034443 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.50%
|
| C000006716 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006716 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
50.00%
|
| C000006716 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
32.10%
|
| C000006716 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006716 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006716 [Member] | Fitch, D Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006716 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006716 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.20%
|
| C000006716 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006716 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.10%
|
| C000006716 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006716 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.20%
|
| C000006711 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006711 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
50.00%
|
| C000006711 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
32.10%
|
| C000006711 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006711 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006711 [Member] | Fitch, D Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006711 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006711 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.20%
|
| C000006711 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006711 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.10%
|
| C000006711 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006711 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.20%
|
| C000119776 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
11.20%
|
| C000119776 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
31.90%
|
| C000119776 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
18.60%
|
| C000119776 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000119776 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000119776 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000119776 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
12.90%
|
| C000119776 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000119776 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000119776 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000119776 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.40%
|
| C000119776 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
97.20%
|
| C000006717 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006717 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
50.00%
|
| C000006717 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
32.10%
|
| C000006717 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006717 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006717 [Member] | Fitch, D Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006717 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006717 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.20%
|
| C000006717 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006717 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.10%
|
| C000006717 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006717 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.20%
|
| C000006718 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006718 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
50.00%
|
| C000006718 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
32.10%
|
| C000006718 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006718 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006718 [Member] | Fitch, D Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006718 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006718 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.20%
|
| C000006718 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006718 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.10%
|
| C000006718 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006718 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.20%
|
| C000006720 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006720 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
50.00%
|
| C000006720 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
32.10%
|
| C000006720 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006720 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006720 [Member] | Fitch, D Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006720 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006720 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.20%
|
| C000006720 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006720 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.10%
|
| C000006720 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006720 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.20%
|
| C000006722 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006722 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
50.00%
|
| C000006722 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
32.10%
|
| C000006722 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006722 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006722 [Member] | Fitch, D Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006722 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006722 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.20%
|
| C000006722 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006722 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.10%
|
| C000006722 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006722 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.20%
|
| C000006723 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006723 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
50.00%
|
| C000006723 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
32.10%
|
| C000006723 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006723 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006723 [Member] | Fitch, D Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006723 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006723 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.20%
|
| C000006723 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006723 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.10%
|
| C000006723 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006723 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.20%
|
| C000006712 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000006712 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
50.00%
|
| C000006712 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
32.10%
|
| C000006712 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000006712 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000006712 [Member] | Fitch, D Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000006712 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006712 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.20%
|
| C000006712 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006712 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.10%
|
| C000006712 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000006712 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.20%
|
| C000070424 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.70%
|
| C000070424 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
50.00%
|
| C000070424 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
32.10%
|
| C000070424 [Member] | Fitch, BB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
7.00%
|
| C000070424 [Member] | Fitch, B Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.90%
|
| C000070424 [Member] | Fitch, D Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.20%
|
| C000070424 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000070424 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
3.20%
|
| C000070424 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000070424 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.10%
|
| C000070424 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
1.80%
|
| C000070424 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
98.20%
|
| C000006745 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
11.20%
|
| C000006745 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
31.90%
|
| C000006745 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
18.60%
|
| C000006745 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006745 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006745 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006745 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
12.90%
|
| C000006745 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006745 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006745 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006745 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.40%
|
| C000006745 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
97.20%
|
| C000006750 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
11.20%
|
| C000006750 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
31.90%
|
| C000006750 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
18.60%
|
| C000006750 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006750 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006750 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006750 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
12.90%
|
| C000006750 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006750 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006750 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006750 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.40%
|
| C000006750 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
97.20%
|
| C000006751 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
11.20%
|
| C000006751 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
31.90%
|
| C000006751 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
18.60%
|
| C000006751 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006751 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006751 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006751 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
12.90%
|
| C000006751 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006751 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006751 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006751 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.40%
|
| C000006751 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
97.20%
|
| C000006744 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
11.20%
|
| C000006744 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
31.90%
|
| C000006744 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
18.60%
|
| C000006744 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006744 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006744 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006744 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
12.90%
|
| C000006744 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006744 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006744 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006744 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.40%
|
| C000006744 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
97.20%
|
| C000006753 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
11.20%
|
| C000006753 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
31.90%
|
| C000006753 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
18.60%
|
| C000006753 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006753 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006753 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006753 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
12.90%
|
| C000006753 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006753 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006753 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006753 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.40%
|
| C000006753 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
97.20%
|
| C000006755 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
11.20%
|
| C000006755 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
31.90%
|
| C000006755 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
18.60%
|
| C000006755 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006755 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006755 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006755 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
12.90%
|
| C000006755 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006755 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006755 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006755 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.40%
|
| C000006755 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
97.20%
|
| C000006756 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
11.20%
|
| C000006756 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
31.90%
|
| C000006756 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
18.60%
|
| C000006756 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006756 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006756 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006756 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
12.90%
|
| C000006756 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006756 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006756 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006756 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.40%
|
| C000006756 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
97.20%
|
| C000006746 [Member] | Fitch, AA Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
11.20%
|
| C000006746 [Member] | Fitch, BBB Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
31.90%
|
| C000006746 [Member] | Fitch, A Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
18.60%
|
| C000006746 [Member] | Fitch, CCC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006746 [Member] | Fitch, CC Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006746 [Member] | Money Market Funds Quality [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006746 [Member] | Moody's, Aaa Rating [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
12.90%
|
| C000006746 [Member] | Not Rated [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.00%
|
| C000006746 [Member] | US Government Debt Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
22.20%
|
| C000006746 [Member] | Money Market Funds [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
2.80%
|
| C000006746 [Member] | Federal Agencies [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
0.40%
|
| C000006746 [Member] | Fixed Income Securities [Member] |
|
| Holdings [Line Items] |
|
| Percent of Total Investments |
97.20%
|