v3.26.1
Shareholder Report, Holdings (Details)
Jun. 30, 2026
C000006467 | L  
Holdings [Line Items]  
Percent of Total Investments 0.40%
C000006467 | I  
Holdings [Line Items]  
Percent of Total Investments 0.40%
C000006467 | F  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006467 | C  
Holdings [Line Items]  
Percent of Total Investments 120.30%
C000006467 | A  
Holdings [Line Items]  
Percent of Total Investments 1.60%
C000006467 | Cash and Equivalents  
Holdings [Line Items]  
Percent of Total Investments (26.50%)
C000006467 | Not Rated  
Holdings [Line Items]  
Percent of Total Investments 3.50%
C000006467 | AssetAgency Adjustable Rate Mortgages  
Holdings [Line Items]  
Percent of Total Investments 0.10%
C000006467 | AssetCommercial Mortgage-Backed Securities  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006467 | AssetU.S. Agency Securities  
Holdings [Line Items]  
Percent of Total Investments 2.70%
C000006467 | AssetMortgages - Other  
Holdings [Line Items]  
Percent of Total Investments 4.40%
C000006467 | AssetAgency Fixed Rate Mortgages  
Holdings [Line Items]  
Percent of Total Investments 18.20%
C000006467 | AssetCollateralized Mortgage Obligations - Agency Collateral Series  
Holdings [Line Items]  
Percent of Total Investments 74.30%
C000155939 | L  
Holdings [Line Items]  
Percent of Total Investments 0.40%
C000155939 | I  
Holdings [Line Items]  
Percent of Total Investments 0.40%
C000155939 | F  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000155939 | C  
Holdings [Line Items]  
Percent of Total Investments 120.30%
C000155939 | A  
Holdings [Line Items]  
Percent of Total Investments 1.60%
C000155939 | Cash and Equivalents  
Holdings [Line Items]  
Percent of Total Investments (26.50%)
C000155939 | Not Rated  
Holdings [Line Items]  
Percent of Total Investments 3.50%
C000155939 | AssetAgency Adjustable Rate Mortgages  
Holdings [Line Items]  
Percent of Total Investments 0.10%
C000155939 | AssetCommercial Mortgage-Backed Securities  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000155939 | AssetU.S. Agency Securities  
Holdings [Line Items]  
Percent of Total Investments 2.70%
C000155939 | AssetMortgages - Other  
Holdings [Line Items]  
Percent of Total Investments 4.40%
C000155939 | AssetAgency Fixed Rate Mortgages  
Holdings [Line Items]  
Percent of Total Investments 18.20%
C000155939 | AssetCollateralized Mortgage Obligations - Agency Collateral Series  
Holdings [Line Items]  
Percent of Total Investments 74.30%
C000006470 | L  
Holdings [Line Items]  
Percent of Total Investments 0.40%
C000006470 | I  
Holdings [Line Items]  
Percent of Total Investments 0.40%
C000006470 | F  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006470 | C  
Holdings [Line Items]  
Percent of Total Investments 120.30%
C000006470 | A  
Holdings [Line Items]  
Percent of Total Investments 1.60%
C000006470 | Cash and Equivalents  
Holdings [Line Items]  
Percent of Total Investments (26.50%)
C000006470 | Not Rated  
Holdings [Line Items]  
Percent of Total Investments 3.50%
C000006470 | AssetAgency Adjustable Rate Mortgages  
Holdings [Line Items]  
Percent of Total Investments 0.10%
C000006470 | AssetCommercial Mortgage-Backed Securities  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006470 | AssetU.S. Agency Securities  
Holdings [Line Items]  
Percent of Total Investments 2.70%
C000006470 | AssetMortgages - Other  
Holdings [Line Items]  
Percent of Total Investments 4.40%
C000006470 | AssetAgency Fixed Rate Mortgages  
Holdings [Line Items]  
Percent of Total Investments 18.20%
C000006470 | AssetCollateralized Mortgage Obligations - Agency Collateral Series  
Holdings [Line Items]  
Percent of Total Investments 74.30%
C000006469 | L  
Holdings [Line Items]  
Percent of Total Investments 0.40%
C000006469 | I  
Holdings [Line Items]  
Percent of Total Investments 0.40%
C000006469 | F  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006469 | C  
Holdings [Line Items]  
Percent of Total Investments 120.30%
C000006469 | A  
Holdings [Line Items]  
Percent of Total Investments 1.60%
C000006469 | Cash and Equivalents  
Holdings [Line Items]  
Percent of Total Investments (26.50%)
C000006469 | Not Rated  
Holdings [Line Items]  
Percent of Total Investments 3.50%
C000006469 | AssetAgency Adjustable Rate Mortgages  
Holdings [Line Items]  
Percent of Total Investments 0.10%
C000006469 | AssetCommercial Mortgage-Backed Securities  
Holdings [Line Items]  
Percent of Total Investments 0.30%
C000006469 | AssetU.S. Agency Securities  
Holdings [Line Items]  
Percent of Total Investments 2.70%
C000006469 | AssetMortgages - Other  
Holdings [Line Items]  
Percent of Total Investments 4.40%
C000006469 | AssetAgency Fixed Rate Mortgages  
Holdings [Line Items]  
Percent of Total Investments 18.20%
C000006469 | AssetCollateralized Mortgage Obligations - Agency Collateral Series  
Holdings [Line Items]  
Percent of Total Investments 74.30%