Shareholder Report, Holdings (Details)	May 31, 2025
C000005523 \| Government National Mortgage Assn.
Holdings [Line Items]
Percent of Net Asset Value	92.90%
C000005523 \| Federal National Mortgage Assn.
Holdings [Line Items]
Percent of Net Asset Value	6.10%
C000005523 \| Federal Home Loan Mortgage
Holdings [Line Items]
Percent of Net Asset Value	4.00%
C000005523 \| OBX Trust
Holdings [Line Items]
Percent of Net Asset Value	1.30%
C000005523 \| Sequoia Mortgage Trust
Holdings [Line Items]
Percent of Net Asset Value	0.60%
C000005523 \| RCKT Mortgage Trust
Holdings [Line Items]
Percent of Net Asset Value	0.50%
C000005523 \| Tricon American Homes Trust
Holdings [Line Items]
Percent of Net Asset Value	0.50%
C000005523 \| Angel Oak Mortgage Trust
Holdings [Line Items]
Percent of Net Asset Value	0.30%
C000005523 \| Finance of America HECM Buyout
Holdings [Line Items]
Percent of Net Asset Value	0.20%
C000005523 \| Morgan Stanley Residential Mortgage Loan Trust
Holdings [Line Items]
Percent of Net Asset Value	0.20%
C000005523 \| U.S. Government & Agency Mortgage-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	104.10%
C000005523 \| Non-U.S. Government Mortgage-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	3.50%
C000005523 \| Other Holdings Grouping
Holdings [Line Items]
Percent of Net Asset Value	(7.60%)
C000190558 \| Government National Mortgage Assn.
Holdings [Line Items]
Percent of Net Asset Value	92.90%
C000190558 \| Federal National Mortgage Assn.
Holdings [Line Items]
Percent of Net Asset Value	6.10%
C000190558 \| Federal Home Loan Mortgage
Holdings [Line Items]
Percent of Net Asset Value	4.00%
C000190558 \| OBX Trust
Holdings [Line Items]
Percent of Net Asset Value	1.30%
C000190558 \| Sequoia Mortgage Trust
Holdings [Line Items]
Percent of Net Asset Value	0.60%
C000190558 \| RCKT Mortgage Trust
Holdings [Line Items]
Percent of Net Asset Value	0.50%
C000190558 \| Tricon American Homes Trust
Holdings [Line Items]
Percent of Net Asset Value	0.50%
C000190558 \| Angel Oak Mortgage Trust
Holdings [Line Items]
Percent of Net Asset Value	0.30%
C000190558 \| Finance of America HECM Buyout
Holdings [Line Items]
Percent of Net Asset Value	0.20%
C000190558 \| Morgan Stanley Residential Mortgage Loan Trust
Holdings [Line Items]
Percent of Net Asset Value	0.20%
C000190558 \| U.S. Government & Agency Mortgage-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	104.10%
C000190558 \| Non-U.S. Government Mortgage-Backed Securities
Holdings [Line Items]
Percent of Net Asset Value	3.50%
C000190558 \| Other Holdings Grouping
Holdings [Line Items]
Percent of Net Asset Value	(7.60%)