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Investments In Available-For-Sale Securities (Contractual Maturities Of AFS Debt Securities At Varying Dates Over The Next 30 Years) (Details) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Jan. 31, 2013
Apr. 30, 2012
Investments In Available-For-Sale Securities [Abstract]    
Span of contractual maturities of AFS debt securities 30 years  
Contractual maturities of AFS debt securities Cost Basis, Less than one year $ 1,000  
Contractual maturities of AFS debt securities Cost Basis, Two to five years 4,192  
Contractual maturities of AFS debt securities Cost Basis, More than five years 386,741  
Contractual maturities of AFS debt securities Cost Basis, Total 391,933  
Contractual maturities of AFS debt securities Fair Value, Less than one year 1,001  
Contractual maturities of AFS debt securities Fair Value, Two to five years 4,526  
Contractual maturities of AFS debt securities Fair Value, More than five years 391,786  
Contractual maturities of AFS debt securities Fair Value, Total $ 397,313 $ 372,352