v3.26.1
Shareholder Report, Holdings (Details)
Apr. 30, 2026
C000006724 [Member] | Fitch, AA Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 1.20%
C000006724 [Member] | Fitch, A Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006724 [Member] | Money Market Funds Quality [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006724 [Member] | Moody's, Aaa Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 2.30%
C000006724 [Member] | Fixed Income Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 99.20%
C000006724 [Member] | US Government Debt Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 89.70%
C000006724 [Member] | Money Market Funds [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006724 [Member] | Federal Agencies [Member]  
Holdings [Line Items]  
Percent of Total Investments 5.70%
C000006728 [Member] | Fitch, AA Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 1.20%
C000006728 [Member] | Fitch, A Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006728 [Member] | Money Market Funds Quality [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006728 [Member] | Moody's, Aaa Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 2.30%
C000006728 [Member] | Fixed Income Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 99.20%
C000006728 [Member] | US Government Debt Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 89.70%
C000006728 [Member] | Money Market Funds [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006728 [Member] | Federal Agencies [Member]  
Holdings [Line Items]  
Percent of Total Investments 5.70%
C000006730 [Member] | Fitch, AA Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 1.20%
C000006730 [Member] | Fitch, A Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006730 [Member] | Money Market Funds Quality [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006730 [Member] | Moody's, Aaa Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 2.30%
C000006730 [Member] | Fixed Income Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 99.20%
C000006730 [Member] | US Government Debt Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 89.70%
C000006730 [Member] | Money Market Funds [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006730 [Member] | Federal Agencies [Member]  
Holdings [Line Items]  
Percent of Total Investments 5.70%
C000006727 [Member] | Fitch, AA Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 1.20%
C000006727 [Member] | Fitch, A Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006727 [Member] | Money Market Funds Quality [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006727 [Member] | Moody's, Aaa Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 2.30%
C000006727 [Member] | Fixed Income Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 99.20%
C000006727 [Member] | US Government Debt Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 89.70%
C000006727 [Member] | Money Market Funds [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006727 [Member] | Federal Agencies [Member]  
Holdings [Line Items]  
Percent of Total Investments 5.70%
C000006726 [Member] | Fitch, AA Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 1.20%
C000006726 [Member] | Fitch, A Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006726 [Member] | Money Market Funds Quality [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006726 [Member] | Moody's, Aaa Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 2.30%
C000006726 [Member] | Fixed Income Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 99.20%
C000006726 [Member] | US Government Debt Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 89.70%
C000006726 [Member] | Money Market Funds [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006726 [Member] | Federal Agencies [Member]  
Holdings [Line Items]  
Percent of Total Investments 5.70%
C000006732 [Member] | Fitch, AA Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 1.20%
C000006732 [Member] | Fitch, A Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006732 [Member] | Money Market Funds Quality [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006732 [Member] | Moody's, Aaa Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 2.30%
C000006732 [Member] | Fixed Income Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 99.20%
C000006732 [Member] | US Government Debt Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 89.70%
C000006732 [Member] | Money Market Funds [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006732 [Member] | Federal Agencies [Member]  
Holdings [Line Items]  
Percent of Total Investments 5.70%
C000006733 [Member] | Fitch, AA Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 1.20%
C000006733 [Member] | Fitch, A Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006733 [Member] | Money Market Funds Quality [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006733 [Member] | Moody's, Aaa Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 2.30%
C000006733 [Member] | Fixed Income Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 99.20%
C000006733 [Member] | US Government Debt Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 89.70%
C000006733 [Member] | Money Market Funds [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006733 [Member] | Federal Agencies [Member]  
Holdings [Line Items]  
Percent of Total Investments 5.70%
C000006725 [Member] | Fitch, AA Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 1.20%
C000006725 [Member] | Fitch, A Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006725 [Member] | Money Market Funds Quality [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006725 [Member] | Moody's, Aaa Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 2.30%
C000006725 [Member] | Fixed Income Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 99.20%
C000006725 [Member] | US Government Debt Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 89.70%
C000006725 [Member] | Money Market Funds [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000006725 [Member] | Federal Agencies [Member]  
Holdings [Line Items]  
Percent of Total Investments 5.70%
C000126364 [Member] | Fitch, AA Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 1.20%
C000126364 [Member] | Fitch, A Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000126364 [Member] | Money Market Funds Quality [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000126364 [Member] | Moody's, Aaa Rating [Member]  
Holdings [Line Items]  
Percent of Total Investments 2.30%
C000126364 [Member] | Fixed Income Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 99.20%
C000126364 [Member] | US Government Debt Securities [Member]  
Holdings [Line Items]  
Percent of Total Investments 89.70%
C000126364 [Member] | Money Market Funds [Member]  
Holdings [Line Items]  
Percent of Total Investments 0.80%
C000126364 [Member] | Federal Agencies [Member]  
Holdings [Line Items]  
Percent of Total Investments 5.70%